Styles made two appearances for the World Wrestling Federation (WWF): on the January 26, 2002, episode of Metal, in a losing effort against The Hurricane, and in a dark match prior to the January 27 episode of SmackDown!, where he was defeated by Rico Constantino.
After weeks of speculation, on January 20, 2016, WWE officially announced that AJ Styles had signed with them. Styles officially made his debut to WWE on January 24 in the 2016 Royal Rumble match, entering at number three. He eliminated Curtis Axel and Tyler Breeze (the latter with help from Roman Reigns) but was later eliminated by Kevin Owens after lasting just under twenty eight minutes in the match. The following night, Styles made his Raw debut where he defeated Chris Jericho via roll-up. After the match, Styles and Jericho shook hands. On the following SmackDown, Styles defeated Curtis Axel after finally using the Styles Clash finisher that was teased at the Rumble and against Jericho. On the February 1 episode of Raw, Styles was the guest on a Miz TV segment. After being interrupted and insulted by The Miz, Styles responded by attacking him, ending the segment. On the February 4 episode of SmackDown, Styles defeated The Miz using the Calf Crusher. After the match, Chris Jericho asked Styles if "he was good enough to beat him twice" and challenged Styles to a rematch next week on the February 11 episode of SmackDown, which Styles accepted via a handshake. On the February 11 episode of SmackDown, Styles and Jericho defeated The Social Outcasts (Adam Rose and Curtis Axel) in an impromptu match; during which both men tried to one-up each other. Later in the main event, Jericho defeated Styles via a Codebreaker. On February 13, Styles once again defeated The Miz at a live event, again by submission. On the February 15 episode of Raw, Styles would once again defeat The Miz via submission, while Jericho watched on commentary. After the match, Styles cut his first official in-ring WWE promo, in which he challenged Jericho to a match at Fastlane, which Jericho called a "phenomenal idea" but seemed reluctant to face Styles a third time. Ultimately, Jericho stated that he would make his decision on the February 18 episode of SmackDown. Jericho would make the match official on the tapings. At Fastlane, Styles defeated Jericho with the Calf Crusher submission. After the match, Styles and Jericho once again shook hands.
After Fastlane, Styles and Jericho formed a tag team, dubbed 'Y2AJ'. On the February 29 episode of Raw, Styles and Jericho defeated WWE Tag Team Champions The New Day in a non-title match. After the match, Styles and Jericho challenged The New Day to a WWE Tag Team Championship match, which WWE confirmed later that night would take place on the March 7 episode of Raw. On the March 7 episode of Raw, Styles and Jericho faced The New Day for the titles, in a losing effort. After the match, Jericho turned heel, attacking Styles with three Codebreakers, after which he shoved a 'Y2AJ' t-shirt into Styles' mouth. Later that night, Jericho explained in an interview with Renee Young that the reason he turned on Styles was out of jealousy as the fans were chanting for Styles and not him. On the following weeks episode of Raw on March 14, Styles got retribution on Jericho after he hit him with the Phenomenal Forearm after Jericho lost a match to Neville via disqualification. Starting on March 17, Jericho would begin intervening in Styles' matches, distracting him each time and ultimately causing Styles to lose. This would first occur on the March 17 episode of SmackDown when Styles faced Intercontinental Champion Kevin Owens in a non-title match, and again on the March 21 episode of Raw, again against Owens. Later that night, Styles would intervene in Jericho's match against Fandango, however Jericho would still win the match. After the match, Styles challenged Jericho to a match at WrestleMania 32, which Jericho rejected on Raw Fallout after the show, saying that Styles "does not deserve to be in the ring" with him ever again. On the March 28 episode of Raw during Jericho's match with Zack Ryder, Styles made an unexpected entrance, demanding Jericho accept the match between the two at WrestleMania. Styles would berate Jericho from ringside during the match, causing him to lose. After the match, Jericho accepted Styles' WrestleMania challenge, making the match official. On April 3 at WrestleMania, Styles was defeated by Jericho.
The following night after WrestleMania 32, on the April 4 episode of Raw, Styles was placed in a Fatal Four-Way Match by interim general manager Shane McMahon to determine the number one contender for Roman Reigns' WWE World Heavyweight Championship. Later that night, Styles would defeat Kevin Owens, Jericho and the returning Cesaro, in which Styles would get revenge on Jericho from WrestleMania by pinning him to become the new number one contender for a shot at the title at Payback. On the following week's episode of Raw, Styles defeated Sami Zayn to keep his title match at Payback a singles match. Had Zayn defeated Styles in this match, he would have earned a spot in the WWE World Heavyweight Championship match at Payback, which would have made it a triple threat match. The following week on the April 18 episode of Raw, Styles met up with the recently-debuted and longtime friends Karl Anderson and Luke Gallows backstage whilst he was being interviewed. Later that night and in the weeks that followed leading up to Payback, several indications (such as Anderson and Gallows attacking Reigns) were made that Anderson, Gallows and Styles were in cahoots with each other in a conspiracy to end Reigns' championship reign at Payback, with Styles vigorously denying the allegations stating that he did not need Anderson and Gallows' help to win the title, with Reigns being sceptical on Styles' allegiance. At Payback, Styles initially defeated Reigns via count out, but the decision was reversed by Shane McMahon and the match was restarted. The match then became a no count outs match, in which Reigns got disqualified by accidentally low-blowing Styles. This would then lead to Stephanie McMahon coming out and restarting the match once again to make it a No Disqualification Match. Shortly into the newly started match, Gallows and Anderson came out and attacked Reigns, giving Styles the advantage. This would prompt The Usos to come out to even the odds. Reigns would eventually defeat Styles without controversy, retaining the title. After the match, a backstage segment took place where it was confirmed that Styles would get his rematch against Reigns for the title at Extreme Rules in an Extreme Rules Match. The following night on Raw, Styles, Anderson and Gallows faced Reigns and The Usos in a six-man tag team match, in a winning effort. Following the match, Gallows and Anderson demanded that Styles use a chair on Reigns, who was defenseless. Styles refused to do so which caused The Usos to attack him, followed by Reigns who proceeded to beat Styles down, culminating in him powerbombing Styles through the announcers table. At Extreme Rules, after interference from Gallows & Anderson and The Usos once again, Styles lost to Reigns.
The following night on Raw after Styles stated that if it wasn't for Anderson and Gallows' interference at Extreme Rules, he would be the WWE World Heavyweight Champion, Anderson and Gallows would come out, which lead to Styles suggesting that the trio part ways for the time being. Anderson and Gallows would then state that they were no longer Styles' friends, signifying the end of The Club. Styles got upset at the rejection to remain "brothers" and told Gallows and Anderson to get out of his ring. Following this, Styles faced Kevin Owens in a WWE World Heavyweight Championship Money in the Bank qualifying match to be inserted into the Money in the Bank match itself at the Money in the Bank pay-per-view, which Styles lost, missing out on a spot in the match.
Following his loss at Extreme Rules, Styles turned his attention away for the WWE World Heavyweight Championship and on the May 30 episode of Raw, welcomed back a returning John Cena, only to attack Cena with help from Anderson and Gallows, turning heel in the process. On June 3, WWE.com announced that Styles would face Cena at Money in the Bank, which Styles won with help from The Club, picking up his first pay-per-view victory since Fastlane. On the July 4 episode of Raw, The Club would attack Cena as Styles stated that they would "beat up John Cena", but was stopped by Enzo Amore and Big Cass, thus leading to a six-man tag team match between the two teams at Battleground. On July 19, at the 2016 WWE draft, Styles was drafted to SmackDown in the first round of picks, being the brand's second pick overall, while Gallows and Anderson were drafted to Raw, thereby splitting up The Club. At Battleground, The Club lost to Cena, Enzo and Cass. On the August 2 episode of SmackDown Live, Styles challenged Cena to another match at SummerSlam, which Cena accepted. Styles would defeat Cena at the event.
Due to his victory at SummerSlam, Styles would immediately re-enter the WWE World Championship picture and would be informed by SmackDown general manager Daniel Bryan on the August 23 episode of SmackDown that he would receive another opportunity at the title at Backlash against champion Dean Ambrose. However, Styles' arrogance led to Dolph Ziggler, who lost a championship match over Ambrose also at SummerSlam, attacking him. Later, Styles defeated Ziggler to prevent the match being turned into a triple-threat match also involving Ziggler. At Backlash, Styles defeated Ambrose to win his first WWE World Championship. On the September 13 episode of SmackDown Live, a returning John Cena interrupted Styles' message to the fans about becoming WWE World Champion, and Cena challenged Styles for the title. Dean Ambrose also appeared and invoked his rematch clause. As a result, SmackDown Live Commissioner Shane McMahon made the main event of No Mercy a Triple Threat match for the WWE World Championship between Styles, Ambrose and Cena. Before that, however, Styles made his first title defense against Ambrose on the September 27 episode of SmackDown Live, in which he successfully did. At No Mercy, Styles was successful in retaining his world championship in the triple threat match after pinning Cena. On the October 11 episode of SmackDown Live, Styles suffered a humiliating non-title match loss to newcomer James Ellsworth, after special guest referee Dean Ambrose made him lose on purpose. Following Ellsworth's victory, Daniel Bryan announced on Talking Smack that Ellsworth had earned a WWE World Championship match against Styles on the October 18 SmackDown Live episode. Styles would go on to lose that match by disqualification after being distracted by Ambrose, thus retaining the title. On the October 25 episode of SmackDown, Ambrose fought Styles in a match for the spot of number-one contender; Ambrose was disqualified when James Ellsworth attacked Styles. A week later, Ambrose won a rematch when Styles was distracted by Ellsworth, allowing Ambrose to score a pinfall, and became number-one contender. Shane McMahon scheduled Styles to defend the championship against Ambrose in a Tables, Ladders, and Chairs match at TLC. Before that, however, Styles teamed with Ambrose as part of Team SmackDown at Survivor Series on November 20, in a winning effort. At TLC, Styles retained his championship against Ambrose after Ellsworth seemingly turned on Ambrose.