Reigns made his NXT debut on the October 31, 2012 tapings by defeating CJ Parker.
Reigns made his main roster television debut on November 18, 2012 at the Survivor Series pay-per-view alongside Dean Ambrose and Seth Rollins, assaulting Ryback during the triple threat main event for the WWE Championship, which led to CM Punk pinning John Cena to retain the title. The trio declared themselves "The Shield" and vowed to rally against "injustice" while denying that they were working for WWE Champion, CM Punk. They would routinely emerge from the crowd to attack Punk's adversaries such as Ryback, The Miz, Kane and Daniel Bryan, who had attempted to save Kane and even Reigns's real life cousin The Rock. This led to a Tables, Ladders, and Chairs match being set up for the TLC pay-per-view pitting the three men of the Shield against Ryback and Team Hell No (Kane and Bryan), which Ambrose, Reigns and Rollins won in their debut match.
The Shield continued to aid Punk after TLC; during Punk and Ryback's TLC match for the WWE Championship on the January 7 episode of Raw, they attacked Ryback, which resulted in Punk retaining his title. During the Royal Rumble event where the Rock challenged for Punk's WWE Championship, match, a blackout occurred and the Rock was attacked in the darkness by unknown assailants, directly leading to Punk pinning Rock; the announcers blamed the Shield for the attack. The match was later restarted with Rock winning. The next day on Raw, the Shield attacked and laid out John Cena; Sheamus and Ryback suffered the same fate when they attempted to save Cena. Later in the show, it was revealed through footage played by Vince McMahon that Punk and/or his manager Paul Heyman had been paying the Shield and Brad Maddox to work for them all along. This set up a six-man tag team match at Elimination Chamber, which the Shield won. The trio defeated the team of Sheamus, Randy Orton and The Big Show at WrestleMania 29. The following night on Raw, The Shield attempted to attack The Undertaker who was paying his respects to late Paul Bearer but were stopped by Team Hell No. This set up a six-man tag team match on the April 22 episode of Raw, where The Shield emerged victorious. Four days later on SmackDown, after Ambrose lost to the Undertaker, he and the rest of the Shield viciously attacked Undertaker, beating him down with chairs and powebombing him through the announcers' table. On the April 29 episode of Raw, The Shield defeated Team Hell No and WWE Champion John Cena in a six-man tag team match, with Reigns pinning Cena for the win. On the May 13 episode of Raw, The Shield suffered their first loss as team in a six-man elimination tag team match by disqualification against John Cena and Team Hell No when Reigns and Rollins, who were already eliminated, attacked Cena while he was performing the STF submission hold on Ambrose, but The Shield still got the upper hand and finished off Cena. On May 19 at Extreme Rules, Reigns and Rollins defeated Team Hell No to win the WWE Tag Team Championship. Reigns and Rollins made their first televised title defense on the May 27 episode of Raw, defeating Team Hell No in a rematch. On the June 14 episode of SmackDown, the Shield's unpinned/unsubmitted streak in televised six-man tag team matches came to an end at the hands of Team Hell No and Randy Orton, when Daniel Bryan submitted Rollins. Three days later at WWE Payback, Rollins and Reigns defeated Bryan and Orton to retain their tag team titles. Rollins and Reigns began feuding with their new number one contenders the Usos, and on July 14, during the Money in the Bank pre-show, defeated them to retain the WWE Tag Team Championship. On September 15, during the Night of Champions pay per view, Rollins and Reigns successfully retained their titles by defeating the Prime Time Players.
Reigns and Rollins managed to lose their tag team titles on October 14 of RAW in a No Disqualification match against the team of Rhodes brothers (Cody Rhodes and Goldust) after Big Show barged in to come to the ring and knocked out Reigns. On October 27 at Hell in a Cell, Reigns and Rollins failed to regain the Tag titles from the Rhodes brothers in a triple threat match, also involving the Usos. At the 2013 Survivor Series event, Reigns was the sole survivor of his team in the traditional 5 on 5 elimination tag team match, last eliminating Rey Mysterio after having pinned three other opponents as well. At TLC: Tables, Ladders & Chairs, CM Punk overcame the Shield in a handicap match after Ambrose received a spear aimed for Punk. On January 6, 2014, Reigns defeated Punk on the special Raw Old School episode, making him the only member of the Shield to have beaten Punk. At the Royal Rumble pay-per-view, Reigns would enter the Rumble match at #15, and set the record for most eliminations in a single Rumble with 12. Reigns eliminated Seth Rollins and Dean Ambrose during the match, possibly turning face. Reigns was the runner-up in the match, being one of the final two wrestlers remaining until being eliminated by the winner Batista. The next night on Raw, The Shield would compete in a six-man tag team match against Daniel Bryan, Sheamus, and John Cena with all three members of the winning team qualifying to compete in the Elimination Chamber match for the WWE World Heavyweight Championship. The Shield lost the match via disqualification after The Wyatt Family interfered and attacked Cena, Bryan, and Sheamus. The Shield wanted revenge and a six-man tag for The Shield against The Wyatt Family at the Elimination Chamber pay-per view was arranged, in which The Shield eventually lost. The next night on Raw, Roman Reigns went one on one with Bray Wyatt, resulting in brawl outside the ring between the remaining members of The Shield and The Wyatt Family, leading Dean Ambrose to cause interference and cost Roman Reigns the win. At Raw on March 3, Rollins walked away by leaving Reigns and Ambrose during their match with The Wyatt Family, causing Wyatt's to win thus causing even more tension in The Shield.
On the March 14th airing of Smackdown, Reigns, along with fellow Shield members Ambrose and Rollins, started a face turn when they stood up to Kane backstage. The official turn happened after they defied Kane yet again, during his match with Big Show, costing Kane the win. Kane pulled Rollins into the ring, but Reigns quickly entered the ring and speared him in the shows final moments before he could chokeslam Rollins, turning them face. Over the next few weeks, The Shield continued exchanging assaults with Kane, who was joined by the New Age Outlaws, leading to a match between the two teams at WrestleMania XXX, which The Shield won. On the April 7 episode of Raw, The Shield officially severed ties with The Authority when Kane revealed that Triple H had coordinated the attack on them several weeks prior. As a result The Shield attacked the entire Authority, protecting new WWE World Heavyweight Champion Daniel Bryan. Over the next few weeks, The Shield competed were heavily outumbered in handicap matches at the order of Triple H.
The Shield defeated Evolution at both Extreme Rules and Payback. The next night on Raw, Batista left Evolution, prompting Triple H to initiate "Plan B"; this involved Rollins turning on the Shield and aligning himself with Triple H. On the June 16 episode of Raw, Reigns would goad Vickie Guerrero, who was left in charge after Triple H and a sick Stephanie McMahon left, to add him to the battle royal to qualify for a spot for the Money in the Bank ladder match for the WWE World Heavyweight Championship, which he won by last eliminating Rusev. At Money in the Bank, Reigns failed to win the title. At the next WWE pay-per-view, WWE Battleground (2014), Reigns took place in the Fatal 4 Way Match for the WWE World Heavyweight Championship match which John Cena won. The following night on Raw, Triple H had planned on announcing Randy Orton as the number one contender for the WWE World Heavyweight Championship, but Reigns attacked Orton, causing Triple H to lose faith in him and replace him in the title match, beginning a feud between the two which culminated with Reigns defeating Orton at SummerSlam.
On September 20, 2014, Reigns was rushed to a Tennessee hospital for emergency surgery on an incarcerated hernia, effectively pulling him from his scheduled match with Seth Rollins at Night of Champions the next night. During his recovery period, he appeared in televised interview segments. Reigns returned to television on the December 8 episode of Raw when WWE awarded him the 2014 "Superstar of the Year" Slammy Award. Reigns made his official return six days later at TLC: Tables, Ladders and Chairs, where he prevented Big Show from attacking John Cena during the latter's match with Seth Rollins, as well as attacking Rollins which allowed Cena to win the match. Reigns would return to the next night on Raw to attack Fandango and Big Show. On the December 22 edition of Raw, Reigns defeated Big Show by count out. The duo would continue their feud in the next year on the January 5 edition of Raw where Reigns once again won via disqualification.
On January 25, 2015, Reigns won the 2015 Royal Rumble match, after entering at number 19 and eliminating six other wrestlers (including Big Show), but Reigns was noted (despite typically being a fan favourite) to have been booed during his entrance, when he won the match, and even when he was endorsed by the Rock after the match, who had helped Reigns fend off Big Show and Kane, similar to last year when Batista (as a babyface) won the 2014 Royal Rumble match but received a negative reaction from the live crowd. On the February 2 episode of Raw, Reigns lost to Big Show in the first time Reigns had lost via pinfall in a one-on-one match on the WWE main roster. Reigns then entered into a feud with Daniel Bryan, who had defeated Rollins on the February 2 Raw to set up a match against Reigns at Fastlane with the winner advancing to face Brock Lesnar in the main event of WrestleMania 31. Reigns defeated Bryan at Fastlane. Bryan endorsed Reigns after the match. At WrestleMania 31, after Reigns had wrestled Lesnar for fifteen minutes in the main event title match, Seth Rollins cashed in his Money in the Bank contract to change the ongoing match into a triple threat. Rollins pinned Reigns to win the WWE World Heavyweight Championship. Reigns would resume his feud with Big Show, leading to Reigns defeating Show in a Last Man Standing Match at Extreme Rules. At Payback, Reigns failed again to win the WWE World Heavyweight Championship against Rollins when he lost a fatal four-way match that included also Ambrose and Orton. At Money in the Bank, Reigns would compete in the Money in the Bank ladder match in which he failed to win after Bray Wyatt interfered, attacking Reigns while he was trying to retrieve the briefcase. Later that night, it was announced that Reigns would face Wyatt at Battleground. Roman would go on to lose the match after interference from Luke Harper. At SummerSlam, Reigns (along with Dean Ambrose) defeated Wyatt and Harper in a tag match. On the August 24 episode of Raw, Reigns and Ambrose fought Wyatt and Harper in a SummerSlam rematch. During the match, Braun Strowman made his WWE debut, joining the Wyatt Family and helping Harper and Wyatt attack Reigns and Ambrose. At Hell in a Cell, Reigns defeated Bray Wyatt in a Hell in a Cell match to end the feud. Reigns then won a fatal 4-way match to become the number-one contender for the WWE World Heavyweight Championship the next night on Raw, but on November 4, at a house show in Dublin, Ireland, then-champion Seth Rollins injured his knee in a match against Kane, vacating the title in the process. A tournament would be held, with the final match taking place a Survivor Series to crown a new champion. Reigns defeated Big Show on the November 9 episode of Raw to advance to the quarterfinals. He then defeated Cesaro the following week to advance to the semi-finals. At Survivor Series, Reigns beat Alberto Del Rio in the semi-finals and then Dean Ambrose to win the tournament and his first WWE World Heavyweight Championship. After being congratulated by Ambrose and then Triple H, Reigns attacked Triple H and, in turn, was attacked by Sheamus who proceeded to cash in his Money in the Bank contract. Sheamus then pinned Reigns, ending his championship reign at just over five minutes.