Grenier auditioned for series one and two of Tough Enough, the reality television program produced by World Wrestling Entertainment in 2001 and 2002, but was unable to attend the selection process due to visa problems. While he was in Florida training and taking part in a modeling shoot, Grenier met WWE producer Pat Patterson, with whom he became good friends. Patterson invited Grenier to attend WWE events for a few tryout matches under the ring name "Sly Grenier". Following a match, Grenier was offered a contract by WWE, and assigned to WWE developmental territory Ohio Valley Wrestling. During this period, Grenier made two appearances for WWE, the first at No Way Out in 2003 as a referee during the match between The Rock and Hulk Hogan in which he helped The Rock to win that match; the other at WrestleMania XIX.
On April 28, Grenier returned to WWE, debuting on the Raw brand alongside fellow French-speaking Canadian René Duprée. The duo were collectively known as La Résistance, and were billed as Frenchmen who were highly critical of American efforts in the War on Terror. Grenier and Duprée made their first appearance on the April 28 edition of Raw as La Résistance by attacking Scott Steiner who had recently made remarks comparing France to hell which offended Grenier and Duprée. La Résistance went on to feud with Scott Steiner and Test (who was being forced to be Steiner's tag team partner by Stacy Keibler) until the 2003 Judgment Day (the team's pay-per-view debut) where La Résistance defeated Steiner and Test.
Grenier and Duprée went on to win the World Tag Team Championship from Kane and Rob Van Dam at the 2003 Bad Blood event. After that the decision was made to add Rob Conway as a third member who began by posing as an American serviceman being abused by Grenier and Duprée until The Dudley Boyz came out to save him. Once the Dudley Boyz had their backs turned to Conway, he attacked them with the American flag they had given him before tearing it off the pole and laying it on top of them. The trio began carrying the French flag to ringside and singing the French national anthem before their matches.
During their time together the trio feuded with several tag teams; including the Dudley Boyz, The Hurricane and Rosey, and Garrison Cade and Mark Jindrak. Grenier suffered back and neck injuries in October 2003, however, forcing Dupree and Conway to hold the group together alone. The trio's eventual reunion was on March 15, 2004 when Grenier returned with a new fauxhawk type haircut. This reunion only lasted a week, as Dupree was drafted to SmackDown! on March 22, 2004 during the Draft Lottery.
Without Dupree, Grenier and Conway continued to team together as La Résistance on Raw. With American-Franco hostility subsiding, WWE ceased billing the team as being from France and instead promoted them as being haughty French Canadians who carried the flag of Quebec to the ring, with Grenier often singing the Canadian national anthem.
The team held the World Tag Team Championship three times. Their first title reign was when they defeated Chris Benoit and Edge on the May 31, 2004 edition of Raw in Grenier's home town of Montreal, Quebec, receiving a huge pop as they beat fellow Canadians Edge and Benoit fair and square. They dropped the belts to them at Taboo Tuesday when Benoit was able to defeat both Grenier and Conway after Edge left him alone in the ring. La Résistance re-captured the titles on the November 1 Raw when Edge walked out on Benoit again. Two weeks later, however, in a three-way tag team elimination match, William Regal and Eugene were the last men standing, beating both La Résistance and the team of Tajiri and Rhyno. Their final Tag Team Title win was at a Raw-brand house show in their home town of Winnipeg, Manitoba, Canada on January 16, 2005. La Résistance defeated William Regal and Jonathan Coachman (Regal's actual partner, Eugene, had been injured at New Year's Revolution. Coachman was forced to partner with Regal, though he was never once tagged in for the entire match). La Résistance dropped the belts a few weeks later on the February 7, 2005 edition of Raw in Tokyo, Japan to William Regal and his new tag team partner Tajiri.
La Résistance repeatedly attempted to regain the gold (mainly on Sunday Night Heat) from Regal and Tajiri in a number of different matches but fell short during each one. During their "last shot" for the gold, the team thought they had won back the titles only to suffer a 'Dusty Finish' as the decision was reversed by referee Mike Chioda due to La Résistance not pinning the legal man. When the match was restarted, the team of La Résistance was unable to gain the victory.
La Résistance were given another chance at Regal and Tajiri during a Tag Team Turmoil match involving five teams during Backlash. In the match, La Résistance managed to eliminate the current champions but were defeated by the last remaining team, Hurricane and Rosey.
The team split in mid-May 2005 after repeatedly quarreling over which of them was the superior wrestler. Grenier and Conway went into singles competition with announcers claiming that the members of La Résistance were trying to "one up" each other in singles matches without the other man in their corner. This saw Conway defeat a jobber and Val Venis, while Grenier was defeated by both Val Venis and Chris Jericho before both men competed in a triple threat match (with Intercontinental Champion Shelton Benjamin for the title), Benjamin won the match after the La Résistance members began to argue and fight with each other. The altercation between the two led to a main event match for the June 12, 2005 edition of Sunday Night Heat (taped June 6). This match saw Conway portray the heel, and defeat Grenier.
Grenier was a last minute trade in the 2005 WWE Draft, forcing him to leave Raw and go to SmackDown ending his already shakey tag team partnership with Conway.After being traded he was not seen on TV for a few months, eventually re-debuting on SmackDown on September 1, 2005. He was repackaged with a gimmick of a narcissistic male supermodel known simply as "Sylvain". A few weeks later he dropped the "I" and was simply known as "Sylvan". He spoke with a light French accent and had a new entrance with techno music, sunglasses, and either a sleeveless white trenchcoat or black pinstripe coat. For his entrance he would enter through the ropes and do a catwalk to the other side in which he would pull his sunglasses down and wink to the crowd on the side of the camera.Sylvan had a small feud with Hardcore Holly holding a victory over the veteran Holly. This feud was used mostly to advance other storylines and saw Sylvan determined to protect his "perfect face" at any cost. Sylvan gained a victory over Holly on the October 7 edition of SmackDown with help from Mr. Kennedy. At No Mercy 2005 after Kennedy defeated Holly, Sylvan came out and continued his feud with him by attacking the injured Holly, which led to Sylvan losing a singles match on an episode of Velocity and a Hardcore Street Fight to the following week on SmackDown, ending an almost 2 month feud. Sylvan entered the 2006 Royal Rumble at No. 10, quickly being eliminated by Bobby Lashley.Sylvan, who had developed a gimmick of running away from a match to protect his face, slowly reverted back to his anti-American gimmick while still acting like a model. Eventually he started stating that his hometown of Quebec was better than anywhere in America and began referring to himself as "Quebec's Ambassador." These gimmicks did not stick, however, and Sylvan soon disappeared off TV. During this time Sylvan would go on to perform in low card situations, often in dark matches before SmackDown tapings and in OVW, while the ambassador gimmick was further developed.On the June 16, 2006 edition of SmackDown vignettes designed to look like vacation promotion videos for Québec and starring Sylvan ("the Ambassador at Large for the Beautiful Province of Québec", or "the Ambassador of Québec" for short) began airing. On July 7 he appeared in-ring to cut a promo attempting to "make peace" with the US fans by welcoming them to visit his home province. The fans responded with a chorus of boos and a loud "USA" chant. In the coming weeks he continued to appear backstage, telling everyone within earshot how much better Québec was than where ever they were from. He made his ring return with this gimmick on the July 28 SmackDown with a win over Tatanka. Sylvan would go on to job against the likes of Tatanka, Vito, and Batista under this gimmick.Sylvan's losing streak would end on the first ever CW Network edition of SmackDown on September 22, 2006, when he defeated Tatanka again by using the ring ropes for illegal leverage. This match itself was most notable for the fact that it took place in Sylvan's hometown of Montreal, Québec, Canada, and Sylvan, who is normally booed, received an overwhelmingly positive reaction from the crowd, including a roaring ovation at almost everything he said and did for the biggest pop of the night. Sylvan was cheered for during the whole match even though he won the match by using the ropes for leverage.The following week on SmackDown, Sylvan defeated Jimmy Wang Yang, who had insulted Canada the week before. The next week, Sylvan cheated again to take a lead in the feud. The following week, Sylvan finally lost to Yang in a six-man tag team match. After disappearing for a couple of weeks, Sylvan finally returned to cost Yang a match versus Gregory Helms. After that match, Matt Hardy came out to help Yang clean house of Helms and Sylvan leading to a tag match the next week. Hardy and Yang won and the feud was seemingly dropped.Sylvan returned to OVW On November 15, making a surprise appearance as "Sylvain Grenier", teaming with Rob Conway to reform their old stable, La Résistance. The reformed team defeated Cody Runnels and Shawn Spears to earn a shot at the OVW Southern Tag Team Titles. However, the team lost the title match. Sylvan then returned to do jobs for Chris Benoit and Vito.On the December 22 edition of SmackDown!, he disguised himself as Santa Claus and attacked World Heavyweight Champion Batista with a steel pipe. This resulted in a match with Batista that Sylvan quickly lost and would end the "Ambassador of Québec" gimmick as Sylvan disappeared from television after this.On January 10, 2007, the Peoria Journal Star reported that Sylvan was pulled over by the police on Interstate 74 around 3pm in Peoria, IL. The report states that Grenier was speeding on his way to the SmackDown!/ECW TV tapings, going 96 mph (154 km/h) in a 55 mph (89 km/h) zone. Grenier was then arrested and booked at the Peoria County Jail. A WWE co-worker paid his $200 bond to get him out of jail at around 4:30pm, just two hours before WWE's TV tapings.On the February 20, 2007 edition of ECW, Sylvan returned and reformed the original La Résistance with René Duprée, defeating a pair of local wrestlers. Due to Duprée getting suspended at the start of March, however, and being sent to rehab after violating the Health and Wellness policy, the team was quietly disbanded as Sylvan began teaming with Rob Conway again in OVW and in dark matches for the SmackDown! brand until Conway was released on May 11, 2007. In July 2007, Sylvan gave himself a new look and was joined by Maryse Ouellet as his valet in dark matches and at house shows.At the August 7, 2007 taping of SmackDown!, Sylvan returned sporting a new look which included freshly bleached blonde hair, his supermodel sunglasses and a gold chain around his neck, in a match against Kane, which he lost. This would turn out to be Grenier's final match for WWE as he was released from his WWE contract six days later.