Duggan was signed to the World Wrestling Federation in January 1987, making his first major appearance with the WWF at WrestleMania III, where he made his pay-per-view debut. In the weeks preceding WrestleMania, Duggan began his first WWF feud with Nikolai Volkoff. At the event, he ran to ringside prior to a match between The Killer Bees and the Iron Sheik and Nikolai Volkoff and interrupted Volkoff's pre-match singing of the Russian national anthem. Duggan had previously done this on TV matches leading up to the event. Although he did not win any major titles in the promotion, for seven years he was a consistent fan favorite with his patriotic gimmick. His character was mainly comical, as he rarely was in major contention for titles. His first major WWF match was at the first-ever Survivor Series pay-per-view in 1987, when he participated in the first Survivor Series match and was a member of the winning team.
Duggan participated in the first-ever Royal Rumble match in 1988, where he was the thirteenth entrant. He won the match (which consisted only of 20 men) after he last eliminated One Man Gang, thus making history as the first-ever Royal Rumble winner and receiving a push. He then started a feud with King Harley Race, whom Duggan defeated several times in both televised and non-televised shows. At WrestleMania IV, he participated in a 14-man tournament for the vacant WWF Championship, where he lost to Ted DiBiase in the first round after interference from André the Giant. Duggan and André started a feud with each other, and attacked each other several times. André interfered in Duggan's match against Hercules on the April 30 edition of Saturday Night's Main Event. Duggan also claimed to be the only wrestler who knocked out André in a WWF ring. Duggan earned a shot at the Intercontinental Championship against The Honky Tonk Man in July 1988, but Honky Tonk got himself disqualified, thus retaining the title.Duggan then began displaying traits of an American patriot by feuding with several foreign villians in the fall of 1988. He got involved in a feud with Dino Bravo, and the two were on opposing sides as Jake "The Snake" Roberts' team fought André the Giant's at the 1988 Survivor Series. Duggan was disqualified after he used his 2x4 on Bravo. Duggan's team eventually lost the match. His next feud was with Russian Boris Zhukov, whom he defeated in a flag match on the November 26 edition of Saturday Night's Main Event. He restarted his feud with Dino Bravo, and at the 1989 Royal Rumble, Duggan and The Hart Foundation (Bret Hart and Jim Neidhart) defeated Bravo and The Fabulous Rougeaus (Jacques and Raymond) in a two out of three falls match. He was involved in a brutal rivalry with Bad News Brown, which culminated in a match at WrestleMania V where both men fought to a no contest.
In mid-1989, he started a feud with King Haku, who won the crown in a battle royal when Harley Race was forced to vacate the crown due to injuries sustained in a match against Hulk Hogan. Duggan defeated Haku to win the crown and earn the title of "King of Wrestling". On the May 25 edition of Saturday Night's Main Event, Duggan earned a shot at the Intercontinental Championship against "Ravishing" Rick Rude. Duggan won the match by countout, and Rude retained the title, as a title cannot change hands via countout or disqualification. At SummerSlam, Duggan teamed with Demolition (Ax and Smash) in a six-man tag team match and defeated André the Giant and the Twin Towers (Akeem and Big Bossman).
He eventually lost his crown to "Macho Man" Randy Savage who began calling himself "Macho King" and started a feud with Savage. At Survivor Series, he captained a team dubbed "The 4x4's" against Savage's team, "The King's Court". Duggan's team went on to lose the match. Duggan started a feud with Big Bossman in the fall of 1989, which culminated in a match at the 1990 Royal Rumble, which Duggan won by disqualification. He faced his former rival, Randy Savage again on the January 27 edition of Saturday Night's Main Event. Duggan faced another former rival, Dino Bravo at WrestleMania VI and went on to win the match. He began teaming with his former rival, Nikolai Volkoff and started a feud with The Orient Express (Tanaka and Sato), whom they beat at SummerSlam. At Survivor Series, he competed on the Hulkamaniacs team captained by Hulk Hogan. Duggan was eliminated but the Hulkamaniacs went on to win the match. At the 1991 Royal Rumble, he participated in the Royal Rumble match, but was eliminated by Mr. Perfect. He began a short feud with WWF Champion Sgt. Slaughter and faced Slaughter for the title on the February 1 edition of The Main Event. Slaughter retained the title by getting himself disqualified. Duggan also served as commentator for the opening match at WrestleMania VII. On the April 27 edition of Saturday Night's Main Event, he participated in a battle royal, won by Intercontinental Champion Mr. Perfect. In late 1991, he patched up things with Sgt. Slaughter, who had turned babyface, and the pair formed a tag team. At Survivor Series, the duo joined the Texas Tornado and Tito Santana in an elimination match to defeat a team led by Col. Mustafa. At the 1992 Royal Rumble, he participated in the Royal Rumble match, which was for the vacant WWF Championship. Ric Flair went on to win the match. On the February 8 edition of Saturday Night's Main Event, Duggan and Slaughter defeated the Beverly Brothers (Blake and Beau). At WrestleMania VIII, he teamed up with Virgil, Sgt. Slaughter, and Big Bossman against The Nasty Boys (Jerry Sags and Brian Knobbs), The Mountie, and Repo Man. Slaughter eventually left active wrestling and Duggan continued to pursue his singles career again.
In mid-1992, Duggan was moved to undercard status. In early 1993, he was involved in a feud with the then near 600 pound Yokozuna, who was the number one contender to the WWF Championship. Manager Mr. Fuji was bad mouthing America and Duggan challenged his protégé to a match. No one at the time had knocked Yokozuna off his feet, and the stipulations to the match were if Duggan knocked Yokozuna off his feet, Duggan would win. On the February 6 edition of Superstars, Duggan won the match after knocking Yokozuna down completely and then was manhandled by the much bigger Yokozuna after the match. Yokozuna gave him about four more Banzai Drops, sidelining Duggan for about four months.
Duggan returned with a new singlet style wrestling attire, and challenged then-Intercontinental Champion Shawn Michaels for the title on the May 3 edition of Monday Night Raw. Michaels ran through the crowd, and retained the title by countout. They had a rematch the following week in a Lumberjack match. Duggan was holding his own in the match and nearly pinned Michaels on several occasions, but was incapacitated after Yokozuna, who was serving as one of the lumberjacks, hit him with a legdrop on the outside of the ring. Duggan ended up losing the match by disqualification after Mr. Perfect, who also was serving as a lumberjack, entered the ring and attacked Michaels while he was pinning the fallen Duggan. At King of the Ring, he wrestled in the King of the Ring tournament, but lost in the first round to Bam Bam Bigelow. Shortly afterwards, Duggan left the WWF.
Duggan signed a contract with World Championship Wrestling (WCW) in late 1994. He debuted in WCW at Fall Brawl, where he defeated Steve Austin in a squash match lasting thirty-five seconds to win the United States Heavyweight Championship to show his respect was to the United States. After becoming champion, he quickly started a feud with Austin, and eventually defeated him in a title rematch at Halloween Havoc by disqualification. The two had a second title rematch on the November 16 edition of Clash of the Champions, where Duggan retained the title again by disqualification. Duggan's reign ended at Starrcade, where he was defeated by Vader.
After he lost the U.S. Title to Vader, Duggan moved to the mid-card division and began wrestling on Saturday Night, where he would later claim that Saturday was "his night", as a way explaining why he appeared almost exclusively on that show. He defeated Bunkhouse Buck at SuperBrawl V. He faced former WWF rival Meng (known as Haku in WWF) in a martial arts match at Uncensored, which Meng won. At The Great American Bash, he defeated Sgt. Craig Pittman by disqualification in Pittman's pay-per-view debut. At Bash at the Beach, Duggan faced another former WWF superstar, Kamala and went on to lose the match.In late 1995, Duggan started a feud with Big Bubba Rogers, which culminated in a Taped Fist match at World War 3, which Rogers won. Duggan participated in the first-ever three ring, 60 man battle royal for the vacant World Heavyweight Championship, but was eliminated. He defeated Loch Ness at a Main Event taping before SuperBrawl VI. At Slamboree: Lord of the Ring, he teamed with VK Wallstreet to compete in tag team qualifying matches to qualify for a battle royal to be crowned "Lord of the Ring". Wallstreet and Duggan defeated the Blue Bloods (Lord Steven Regal and Squire David Taylor) before losing to Dick Slater and Earl Robert Eaton. Duggan then started a short feud with upcoming superstar Diamond Dallas Page, to whom he lost in a Taped Fist match at Bash at the Beach.Duggan had an encounter with former Slamboree partner VK Wallstreet on the August 15, 1996 edition of Clash of the Champions, which Wallstreet won. At World War 3, he unsuccessfully participated in a three ring 60 man battle royal to determine the #1 contender to the WCW World Heavyweight Championship. Duggan had no major pay-per-view matches for over two years before he disappeared in 1998 after he was diagnosed with kidney cancer.
During this period, he found the World Television Championship in a trash can, which had been thrown away by the previous champion Scott Hall earlier and claimed it for himself. He would make title defenses exclusively on Saturday Night, and remained champion until the title was deactivated. During this time, Duggan lost confidence in himself as a wrestler. On an episode of Nitro, Duggan was attacked by a then heel Goldberg. Duggan then challenged Goldberg to a match and nearly defeated him before falling victim to the spear and Jackhammer. After WCW was rebooted and the nWo storyline ended, it was replaced by two new factions in WCW called the Millionaire's Club and the New Blood, which began feuding with one another. The Millionaire's Club consisted of veterans and the New Blood consisted of young wrestlers of the 1990s. Duggan joined the Millionaire's Club and helped them in their war against the New Blood.
Near the end of his career in WCW, he became involved in more major storylines, including one that involved him turning heel by abandoning the patriotic gimmick for a short time to join Lance Storm and his faction Team Canada. This was the first time Duggan worked as a heel since breaking away from the Rat Pack in Mid-South Wrestling. This angle began when Duggan started working with the Misfits In Action (MIA) but at Fall Brawl, he turned on MIA's leader U.S. Champion General Rection during Rection's title defense against Team Canada's Lance Storm. Duggan was the special guest enforcer for the match, and helped Storm in winning the match. At Halloween Havoc, Duggan and Storm teamed up against General Rection in a handicap match for Storm's U.S. Title. Duggan missed an attack on Rection, which led to Rection pinning Duggan to win the match. Duggan continued to unintentionally cost Team Canada many of their victories, but he helped Storm to defeat The Cat in a match at Starrcade. However, after the match, Team Canada turned on Duggan by attacking him. Duggan then began feuding with Team Canada and at Sin, he was the special guest referee in a Penalty Box match between Team Canada and the Filthy Animals. Despite Duggan's role as the referee, Team Canada won the match after Storm forced Kidman to submit.
Duggan appeared on the October 3, 2005 edition of Raw, dubbed "The Homecoming" along with several other WWF/E alumni. He and the other legends would end up attacking Rob Conway after Conway came to the ring and insulted them. He was also one of the "legends" available for fans to select to team with Eugene in a match at Taboo Tuesday to face the team of Conway and Tyson Tomko (Jimmy Snuka was chosen instead of Duggan). He returned to WWE again after Royal Rumble, with a message from John Cena to Lita after Edge lost the WWE Championship: "HOOOOO!" Duggan and Edge began a short feud with each other which culminated in a match that occurred two weeks later on Raw, which Edge went on to win after Lita provided a distraction.After feuding with Edge and Lita, he became mentor and tutor to Eugene and the two feuded with the Spirit Squad. He began working on Heat but also made a few appearances on Raw with Eugene and The Highlanders. On the November 6 edition of Raw, Duggan and Eugene faced the Spirit Squad in a match with the stipulation that the losing team would disband and never team up again. After Duggan was caught off guard after the other Spirit Squad members interfered and then pinned for the win, Eugene snapped and attacked "Hacksaw", signaling Eugene's first heel turn in WWE. He started a feud with Eugene, and three weeks later, the two had a match with each other, which Eugene easily won. On the December 18 edition of Raw, Duggan participated in a number one contender's battle royal with the winner facing WWE Champion John Cena in the main event for the WWE Title, which was won by Duggan's former rival Edge.
At New Year's Revolution, Duggan teamed with Super Crazy in a tag team turmoil match to determine the number one contenders to the World Tag Team Championship, in a losing effort. On the January 15 edition of Raw, Duggan was dragged out by Rated-RKO (Edge and Randy Orton) and was beaten to a pulp. Edge and Orton feared Duggan would help out Shawn Michaels in a handicap match (referencing, without explanation for viewers, the fact that both Duggan and Michaels were in Mid-South Wrestling together 20 years earlier) and wanted to set an example for anyone else backstage that wished to do the same. On the March 5 edition of Raw, he faced Chris Masters in a Masterlock Challenge, but was unable to break the hold.
After months of wrestling on Heat, Duggan returned to Raw in July, teaming up with The Sandman and feuding with Carlito and William Regal. This team was short lived as Duggan soon returned to Heat and Sandman was soon released from his contract with WWE. On the August 6 edition of Raw, Duggan was a contestant in the WWE Dating Game, a parody of The Dating Game. However, Duggan lost when Maria chose Ron Simmons. On September 17, Duggan suffered a knee injury.
On December 31, he lost a 2008 Royal Rumble qualifying match to the returning Umaga. After that, he began primarily appearing on Heat once again, defeating locals and teaming with Super Crazy. On the March 17 edition of Raw, he participated in a 17-on-2 handicap elimination tag team match as part of the 17 members of the Raw roster against John Cena and Randy Orton. After Cody Rhodes was eliminated, all the remaining Raw superstars attacked Cena and Orton, resulting in their disqualification. Duggan also took part in a 12-on-12 tag team preview match on the March 25 edition of ECW. The match was a preview for a WWE.com exclusive match, a 24-man battle royal held just before WrestleMania XXIV aired to determine the #1 contender for an ECW Championship match. The match was eventually won by Kane.
In the summer of 2008, Duggan started a short feud with Cody Rhodes and Ted DiBiase after Rhodes and DiBiase tried to convince Duggan to retire and let the young talent run the show. Soon after, Duggan would come out and announce that he would take Rhodes and DiBiase's advice and retire, but Jerry "The King" Lawler stopped him by convincing Duggan to stay. Duggan and Lawler then wrestled Rhodes and DiBiase on the edition of Raw, but lost the match. On the edition of Raw, Duggan confronted Santino Marella and Beth Phoenix during one of Marella's tirades against his three possible opponents at Cyber Sunday (Roddy Piper, Goldust, and The Honky Tonk Man). Upset at being interrupted, Marella took off his clothes, getting ready to fight Duggan. Yet before a fight could begin, Marella instructed Phoenix to throw the clothes in Duggan's face, a move that temporarily distracted Duggan. Marella then smashed a guitar over Duggan's head, ending the segment.
In 2009, Duggan appeared in the 2009 Royal Rumble match, entering at #29 and setting a record for the most years between Royal Rumble appearances. He was eliminated from the match by The Big Show. Since then, he was involved in the WWE.com exclusive show, Top Rope Theater, which ended when Cryme Tyme was drafted to SmackDown in 2009. After Top Rope Theater, Duggan began wrestling on Raw house shows but never reappeared on television. On the August 10 episode of Raw, Duggan made his final televised appearance as Sgt. Slaughter had him come out in place of Bret Hart, due to the episode taking place in Canada, when he introduced "The best there was, the best there is, and the best there ever will be" in order to mock the Canadian fans.He was soon released from the WWE.