After failing his audition for Tough Enough 2, he was accepted as a cast member for Tough Enough III in 2002, eventually becoming the co-winner with Matt Cappotelli. He competed in a Bar Room Brawl match at Vengeance in July 2003. Morrison and Cappotelli made an appearance on an episode of Heat in January 2004, losing to Garrison Cade and Mark Jindrak in a tag team match.
On March 1, 2004 Hennigan debuted on the Raw brand under the ring name "Johnny Blaze". His gimmick was that he was the apprentice and assistant of the Raw General Manager, Eric Bischoff. The next week his name was changed to "Johnny Spade", and three weeks after that it was changed to "Johnny Nitro". Nitro acted as Bischoff's apprentice and assistant until June, when he disappeared from television. To explain his departure, Hennigan lost a match on the June 7 episode of Raw against Eugene, which carried the stipulation that upon his loss, he also lost his job.
Nitro (along with Joey Mercury and Melina) were called up to the main SmackDown! roster in April 2005. In their first match for the brand, they won the WWE Tag Team Championship from Rey Mysterio and Eddie Guerrero. MNM held the titles for three months, having successful titles defenses against the teams of Mysterio and Guerrero and Hardcore Holly and Charlie Haas, before dropping them to the Legion of Doom (LOD) at The Great American Bash. They won the titles again in a four-way match defeating The Mexicools, William Regal and Paul Burchill, and LOD, but lost them to Mysterio and Batista days before Armageddon in December. Due to help from Mark Henry, MNM defeated Mysterio and Batista in a rematch on December 30 to win the championship for the third time. They began a rivalry with the team of Paul London and Brian Kendrick that lasted over three months, until they dropped the championship to London and Kendrick at Judgment Day in May, with the storyline leading to Nitro and Melina suddenly turning on Mercury after the match, breaking up the group. Later that night, Melina and Nitro were booked off the SmackDown! brand by having the General Manager, Theodore Long, "fire" them.
The next week Nitro, along with Melina, debuted on the Raw brand in a loss to WWE Champion John Cena. Nitro was immediately placed in the WWE Intercontinental Championship picture, beginning his first reign at Vengeance in June defeating champion Shelton Benjamin in a triple threat match also involving Carlito. Nitro held the belt for four months, feuding with several challengers before losing it to Jeff Hardy on the October 2 episode of Raw. Nitro recaptured the Intercontinental Championship from Hardy on November 6 when the General Manager for the night, Eric Bischoff, restarted a match that Nitro lost by disqualification. The next week, however, Hardy won the title back.
In addition, Nitro was featured in a sporadic feud with John Cena throughout 2006. The feud was unique in that it also involved non-wrestler Kevin Federline, who appeared on the October 16 episode of Raw. Federline was then, in storyline, trained by Nitro for a match with Cena, which he won after interference from Umaga.
In November, MNM reunited on an episode of Raw to accept an open challenge from The Hardys (Jeff and Matt Hardy) at December to Dismember, a match that they lost. MNM and the Hardys feuded across the brands throughout December, with the feud intensifying after Mercury was legitimately injured at Armageddon in a four-way ladder match. Mercury wrestled Matt on SmackDown!, while Nitro continued to feud with Jeff for the Intercontinental Championship on Raw, including a steel cage match at New Year's Revolution. MNM lost to The Hardys at the Royal Rumble, and again at No Way Out in February which ended the feud. MNM continued on and off as a team until Mercury was released from WWE in March 2007.
Without Mercury, Nitro's association with Melina was played down as she began competing for the WWE Women's Championship. Nitro was paired up with Kenny Dykstra, until the 2007 WWE Draft, during which Nitro was sent to the ECW brand and Dykstra to SmackDown!.
Nitro made his ECW debut on June 19 with a win over Nunzio, and the next week, at Vengeance: Night of Champions, took the vacant ECW World Championship in a match against CM Punk, when he was called upon to replace the missing Chris Benoit.
A few weeks after getting the title, Nitro's name was changed to John Morrison and his gimmick was tweaked to be even more conceited and self-aggrandizing. After defeating CM Punk again at The Great American Bash, he began taking part in a "15 Minutes of Fame" segment on the weekly ECW on Sci Fi television program, where if a wrestler could either defeat him or last 15 minutes without being defeated, they would earn a title shot. The first wrestler to do so was CM Punk, which set up another match between the two at SummerSlam. Morrison won by illegally using the ring ropes for leverage. The next week, Punk again became the number one contender; this time, however, Punk won the championship. Morrison was then suspended for thirty days for violating WWE's Wellness Program, renewing his feud with Punk upon his return, as well as competing against The Miz for the number one contendership to the title.
Though rivals, Morrison and The Miz were placed as a team on the November 16 episode of SmackDown!, and were booked to take the WWE Tag Team Championship from the imploding team of Matt Hardy and Montel Vontavious Porter. At Survivor Series, Morrison and The Miz were both defeated by CM Punk in a triple threat match for the ECW Championship. After the defeat, the rivalry aspect of Miz and Morrison's relationship faded, as they were now portrayed as trusting friends. In February 2008, Morrison and The Miz were given a streaming segment on WWE.com named The Dirt Sheet in which they mocked other wrestlers and facets of pop culture. The team had many successful title defenses over the next few months, before dropping the titles to Curt Hawkins and Zack Ryder at The Great American Bash in a fatal four-way match which also featured Jesse and Festus and Finlay and Hornswoggle. Neither The Miz nor Morrison were pinned, as Hawkins pinned Jesse to win the titles.
Afterwards, they started a feud with Cryme Tyme (Shad Gaspard and JTG) through each team's Internet show, which then transitioned to matches on Raw and at Cyber Sunday. After winning the Slammy Award for Best WWE.com exclusive earlier in the day, Morrison and The Miz won the "Tag Team of the Year" Slammy Award on the December 8, 2008 episode of Raw. On December 13, Morrison and The Miz won the World Tag Team Championship from CM Punk and Kofi Kingston at a Raw live event. Morrison and the Miz engaged themselves in a feud with The Colóns (Carlito and Primo), who were the reigning WWE Tag Team champions which resulted in both teams defending their respective championships successfully in separate matches. Their feud culminated in a tag team lumberjack match at WrestleMania XXV where both titles were on the line, but it was The Colóns who would unify the championships with a win. As part of the 2009 WWE Draft on April 13, The Miz was drafted to the Raw brand to split up the team, and, as a result, Miz attacked Morrison.
On April 15, 2009, Morrison returned to the SmackDown brand as part of the 2009 Supplemental Draft. In Morrison's first match back for the brand, on April 17, he defeated R-Truth. On the May 1 episode of SmackDown, Morrison engaged in a backstage fight with Chris Jericho, after Jericho slapped him, thus turning into a face (fan favorite character). At Judgment Day, Morrison defeated Benjamin in a rematch. On the July 31 episode of SmackDown, Morrison had his first World Heavyweight Championship match against champion Jeff Hardy, but failed to win the title. On the September 4 episode of SmackDown, Morrison defeated Rey Mysterio to win his third Intercontinental Championship. At the TLC: Tables, Ladders & Chairs pay-per-view, Morrison lost the Intercontinental Championship to Drew McIntyre.
In 2010, Morrison formed a friendship with R-Truth. He won an Elimination Chamber pay-per-view qualifying match and participated but lost and failed to capture the World Heavyweight Championship. Morrison and Truth defeated Cryme Tyme and The Hart Dynasty to earn a Unified WWE Tag Team Championship match at WrestleMania XXVI against ShoMiz (The Big Show and The Miz). At the event, they failed to win the championship.
On April 26, Morrison was drafted to the Raw brand as part of the 2010 WWE Draft, and lost his first match back for the brand later that night to Jack Swagger. Later that week, in his final match on SmackDown, he lost to Cody Rhodes. On June 1, Morrison announced that he would be mentoring Eli Cottonwood in the second season of WWE NXT. Cottonwood, however, was the second person eliminated from the competition, and was eliminated on the July 27 episode of NXT.
Morrison then went on to participate in the main event of SummerSlam in which he and six teammates were victorious in a seven-on-seven elimination tag team match against The Nexus. At Survivor Series, Morrison beat Sheamus. In November 2010, Morrison entered the 2010 edition of the King of the Ring tournament. He defeated Tyson Kidd, Cody Rhodes, and Alberto Del Rio en route to the final where he lost to Sheamus. He continued to feud with Sheamus, and defeated him in a WWE Championship number one contender's ladder match at the Tables, Ladders & Chairs pay-per-view on December 19. Morrison received his WWE Championship match on the January 3, 2011 episode of Raw, but lost to reigning champion The Miz in a falls count anywhere match.
On January 30, Morrison participated in the Royal Rumble match at the Royal Rumble event and despite not winning, he had a stand-out moment when Morrison was knocked off of the ring apron and performed a cat leap onto the barricade to avoid having his feet touch the floor and then went on to precision jump onto the ring steps and re-enter the ring. On February 20, Morrison participated in the Raw Elimination Chamber match at the Elimination Chamber pay-per-view event but was eliminated by CM Punk. Critics once again praised Morrison's Parkour skills during the Chamber match, during which Morrison climbed to the top of the cage while inside the cage before dropping to deliver a crossbody on and eliminate Sheamus. At WrestleMania XXVII, Morrison teamed with Trish Stratus and Snooki in a mixed tag match to defeat the team of Dolph Ziggler, Michelle McCool and Layla.
Morrison participated in a five-man gauntlet match for the number one contender's spot for the WWE Championship on the April 11 episode of Raw, but was eliminated by R-Truth. The next week on Raw, Morrison challenged R-Truth to a match later on in the night, with the stipulation that if Morrison won, he would take R-Truth's place in the steel cage match at Extreme Rules. Truth accepted, and lost the match, making Morrison the new number one contender. At Extreme Rules, R-Truth interfered in the match, attacking Morrison and preventing him from winning the match. The next night on Raw, Morrison was attacked by Truth again, prior to a scheduled match between the two which did not happen as a result. Morrison then underwent surgery to correct a legitimately pinched nerve in his neck.
Morrison initially returned on the June 13 episode of Raw, but R-Truth attacked him again prior to their scheduled match, re-injuring him. Morrison returned again on the July 25 episode of Raw, attacking R-Truth. At SummerSlam, Morrison competed in a six-man tag team match, teaming with Kofi Kingston and Rey Mysterio to defeat R-Truth, Alberto Del Rio and The Miz. The following night on Raw, Morrison defeated R-Truth in a Falls Count Anywhere match to end their feud. At Night of Champions, Morrison competed in a Fatal-4-Way match for the WWE United States Championship, also involving Alex Riley, Jack Swagger and Dolph Ziggler, in which Ziggler successfully retained his title. Morrison then challenged Rhodes for the Intercontinental Championship twice, first on the September 26 episode of Raw in a 10 man battle royal and later at the Hell in a Cell pay per-view but failed to win on both occasions. Following this he then went on a losing streak, lasting a number of weeks. On the November 7 episode of Raw, Morrison picked up his first victory in three months, over United States Champion Dolph Ziggler in a non-title match, ending the losing streak. He faced Dolph Ziggler for the WWE United States Championship at Survivor Series but lost via pinfall.
Morrison's last advertised WWE appearance on the November 28 episode of Raw, Morrison faced his former tag team partner The Miz in a Falls Count Anywhere match. The Miz won the match by delivering his Skull-Crushing Finale maneuver on the steel stage. This was used as an injury angle to write Morrison off television, as Morrison was stretchered away after the match. The next day, WWE acknowledged the end of Morrison's contract.