In June 2002, Lesnar won the King of the Ring tournament, defeating Rob Van Dam in the final round. This victory earned him a shot at the WWE Undisputed Championship at SummerSlam. On July 22, Lesnar joined the SmackDown! brand. After a quick feud with Hollywood Hulk Hogan in August 2002, Lesnar began feuding with the Undisputed Champion The Rock. At the main event of SummerSlam, Lesnar defeated The Rock to become the WWE Undisputed Champion. With his victory at age 25, Lesnar became the youngest WWE Champion in history (a record that was previously held by The Rock and stands to date). With the WWE Undisputed Championship being defended on both brands, Raw General Manager Eric Bischoff expected Lesnar to be able to return to Raw the following night. However, SmackDown General Manager Stephanie McMahon announced that Lesnar's contract only required him to defend the title on SmackDown!, forcing Bischoff to establish a new world championship for his brand. With the new World Heavyweight Championship on the Raw brand, the now disputed WWE Undisputed Championship simply became known as the WWE Championship.
Lesnar's rapid rise to the top of WWE in 2002 led to a feud with The Undertaker, which involved a match at Unforgiven. It ended in a double disqualification leading to Lesnar retaining the title. The next month, at No Mercy, he faced The Undertaker again, this time in a Hell in a Cell match. Leading up to the match, Lesnar broke the Undertaker's hand with a propane tank. Despite Heyman begging McMahon not to let The Undertaker use his cast as a weapon, the request was denied and the match went on as planned. In a match that saw both wrestlers and even Heyman covered in blood, it ended when Lesnar reversed an attempted Tombstone piledriver into his finishing F-5 maneuver for the win. Six days after his Hell in a Cell match with The Undertaker, Lesnar successfully retained his WWE title in a Handicap match with Heyman at the Rebellion pay-per-view against Edge.
Lesnar's next opponent was The Big Show. Heyman was convinced more than anyone that Lesnar could not win, trying to talk him out of defending the title. Lesnar refused and lost the championship in Madison Square Garden at the Survivor Series pay-per-view to Big Show, Lesnar's first pinfall loss in WWE. When the champion tried to pin the 500-pounder following an F-5, he was betrayed by Heyman who pulled the referee out of the ring allowing Big Show to capitalize and win the title. As a result, Lesnar became a fan favorite. Following Survivor Series, Heyman made it clear that Lesnar would not get a rematch, even going so far as to sneak a special clause in his contract. At the Royal Rumble event in January 2003, Lesnar defeated The Big Show in a quallification match. Lesnar later entered the Rumble as the No.29 entrant where he eventually won by eliminating The Undertaker.
After winning the Royal Rumble match, Lesnar spent the next two months in a scripted feud with WWE champion Kurt Angle, who Lesnar had helped win the title and who was represented by Paul Heyman. Angle had Heyman, and Team Angle (Shelton Benjamin and Charlie Haas) behind him, but Lesnar overcame these opponents. Lesnar regained the WWE Championship in the main event of WrestleMania XIX. Toward the end of the match, he botched a shooting star press; in this match he overestimated the distance he was capable of jumping for the move and under-rotated, slamming his head into Angle's side and ribcage. This stunned Lesnar and forced Angle to improvise the finish of the match. Lesnar suffered a severe concussion from the botched move.
Following WrestleMania, John Cena began targeting Lesnar for almost ending his career (by using the F-5 to propel his leg into a ring post) after a previous match between the two. This led to Cena receiving a title match at Backlash in which Lesnar successfully retained the title. Earlier at the Backlash pay-per-view, Big Show had injured Rey Mysterio badly, resulting in Mysterio being carried out on a stretcher. This injury led to Lesnar renewing his feud with Big Show, which led to a Stretcher match at Judgment Day for the title. Lesnar successfully retained his title with help from Rey Mysterio and a forklift. During the scripted rivalry, on SmackDown!, Lesnar lifted Big Show over in a Superplex, which caused the ring to collapse on impact. At Vengeance, Lesnar lost his title to Kurt Angle in a no disqualification Triple Threat match that involved Big Show.
Prior to SummerSlam, Lesnar became a villain by betraying Kurt Angle, leading to a rematch at the event. Lesnar lost to Angle when Angle made Lesnar tap out to the ankle lock. A second rematch between Lesnar and Angle, an Iron Man match, was later held on an episode of SmackDown!. Lesnar defeated Angle in a match that was said as one of the best in SmackDown! history. Angle slapped on the ankle lock with a few seconds remaining but Lesnar did not tap. Lesnar won with five falls to Angle's four, making him a three-time WWE Champion. Lesnar's first challenger for his newly won title was The Undertaker. Lesnar had previously cost Undertaker the title in a match against then-champion Kurt Angle, which granted him a shot at Lesnar's title. At No Mercy, Lesnar defeated Undertaker in a Biker Chain match. Lesnar realigned himself with Paul Heyman after Heyman became General Manager of SmackDown! and formed Team Lesnar with Big Show, Matt Morgan, A-Train, and Nathan Jones for a 10-man Survivor Series match at Survivor Series. In the climax of the match, Chris Benoit became the second person to make Lesnar tap out. Lesnar faced Benoit in a singles bout two weeks later for the WWE Championship on SmackDown!, where Lesnar won after Benoit passed out to Lesnar's debuting Brock Lock submission hold.
The Survivor Series event marked the first time Lesnar met Goldberg from the Raw brand, a man to whom Lesnar had been compared due to their similar physiques and undefeated streaks in wrestling. Lesnar claimed in a backstage interview that he could beat anybody in the world, leading to Goldberg interrupting the interview and introducing himself to Lesnar, shaking hands with him before leaving with a staredown. Lesnar followed this rivalry with a feud involving Hardcore Holly. In this storyline, Holly wanted revenge on Lesnar for legitimately breaking his neck during a previous match between the two in 2002. During that match, Lesnar went for a powerbomb, but Holly, who has gained a reputation of being uncooperative with rookies, supposedly sandbagged mid-lift so that Lesnar could not get him all the way up. Holly compounded the difficulty of the move by not lifting up at the waist. During the struggle, Lesnar ended up dropping Holly on his head, leaving him in need of neck surgery and out of action for a year. At the Royal Rumble in 2004, Lesnar defeated Holly to retain the WWE Championship. Later in the Royal Rumble match, Lesnar attacked Goldberg and delivered an F-5, enabling Kurt Angle to eliminate him.
In February, Lesnar faced Eddie Guerrero for the title at No Way Out. During the match, Goldberg interfered in the match, allowing Guerrero to capitalize and pin Lesnar after a frog splash to win the WWE Championship. The match set up an interbrand match for WrestleMania XX between Lesnar and Goldberg. During the feud with Goldberg, Lesnar was feuding with Raw's Stone Cold Steve Austin (who was shown suggesting to Goldberg that he attack Lesnar at No Way Out). When Lesnar attacked Austin on Raw and stole his four-wheeler, Austin was inserted as the special guest referee for the WrestleMania match. Behind the scenes, it was widely known that the match would be Goldberg's last in WWE. Only a week before WrestleMania, however, rumors surfaced that Lesnar, too, was leaving in order to pursue a career in the National Football League. As a result, Lesnar's match with Goldberg became a fiasco as the fans at Madison Square jeered both wrestlers with very loud and distinct chants of "na na, hey hey, goodbye" and "you sold out," with the crowd support mostly given to special referee Austin. Although Goldberg won the match, both men were given a Stone Cold Stunner by Austin on their way out.
On April 2, 2012, Lesnar returned to the WWE at the end of Monday Night Raw, attacking John Cena. The following week, general manager John Laurinaitis introduced Lesnar as the new face of the WWE, he then announced Lesnar would face Cena at the Extreme Rules PPV. Cena then interrupted and slapped Lesnar in the face, the two then brawled which resulted in Cena getting a busted lip. On April 16, it was announced, via the official WWE website, that Lesnar would face Cena in an Extreme Rules match. At the pay-per-view, Lesnar bloodied Cena with MMA-style strikes, but would ultimately lose the match.
The following night on Raw, Chief Operating Officer Triple H returned to confront Lesnar over his new contract demands, saying wouldn't be authorizing any of them. Lesnar would then attack Triple H, apply a Kimura lock and break his arm. On the May 7 episode of Raw, Paul Heyman, serving as legal representative for Lesnar, announced that Lesnar had (kayfabe) quit the WWE. The following week, Heyman announced that Lesnar was filing a lawsuit against WWE for breach of contract. At the No Way Out pay-per-view in June, Triple H challenged Lesnar, who was not present, to a match at SummerSlam, which Heyman refused on Lesnar's behalf the following night on Raw. On July 23, during the 1,000 episode Raw, Heyman accepted Triple H's challenge after his wife Stephanie McMahon assaulted him. Following this, Lesnar returned and had a brief brawl with Triple H before he was chased from the ring. In a psychological bid, Lesnar would break the arm of Triple H's best friend Shawn Michaels with the Kimura Lock on the August 13 episode of Raw. Six day later at SummerSlam, Lesnar defeated Triple H via submission. The next night on Raw, Brock Lesnar and Paul Heyman released a Tout saying he was once again leaving WWE.
Lesnar returned to Raw on January 28, 2013, confronting Vince McMahon, who was just seconds away from firing Paul Heyman. Despite Heyman's pleas, Lesnar hit McMahon with an F-5, effectively breaking McMahon's pelvis. The following week, Vickie Guerrero revealed herself to be the one who signed Lesnar to a new contract. Afterwards, Lesnar confronted and attacked The Miz. On February 25, Lesnar interrupted a match between Paul Heyman and Vince McMahon, only to get into a brawl with a returning Triple H which resulted in Lesnar having his head split open and requiring 18 stitches. The following week, Triple H issued a challenge to Lesnar, requesting a rematch with him at WrestleMania 29, which Lesnar accepted but only after Triple signed a contract and Lesnar named the stipulation. The following week, after Triple H signed the contract and assaulted Heyman, the stipulation was revealed as No Holds Barred with Triple H's career on the line. Lesnar ended up losing the match after Triple H hit him with a Pedigree onto the steel steps. On the April 15 episode of Raw, Lesnar attacked 3MB (Heath Slater, Drew McIntyre, and Jinder Mahal) before Paul Heyman challenged Triple H to face Lesnar in a Steel Cage match at Extreme Rules, which Triple H accepted the following week. On May 19 at the pay-per-view, Lesnar defeated Triple H in the steel cage thanks to interference from Heyman. Lesnar returned on the June 17 episode of Raw, giving an F-5 to CM Punk. Despite the accusations from Punk, Heyman claimed that he was not behind Lesnar's attack on him. However, in July, Heyman turned on Punk, and claimed that Punk could not beat Lenar, which led to Lesnar making his return and attacking Punk on the July 15 episode of Raw. The following week on Raw, Punk challenged Lesnar to a match at SummerSlam, which Heyman accepted on Lesnar's behalf. On the August 5 episode of Raw, Lesnar came out and again brawled with Punk; Lesnar ultimately got the upper hand on Punk, despite Punk putting up a valiant effort. Punk then got revenge the following week by attacking Lesnar with a camera and a steel chair after Heyman tried to trap Punk by having him fight both him and Lesnar instead of the scheduled match of Punk versus Heyman. Six days later at SummerSlam, Lesnar defeated Punk in a No Disqualifications match. On the December 30 edition of Raw, Lesnar returned with Paul Heyman to announce his intentions to challenge the winner of the upcoming WWE World Heavyweight Championship match between Randy Orton and John Cena at the Royal Rumble. Lesnar then called out any wrestler who wanted to stop him, which resulted in Mark Henry answering the challenge and the two brawling until Lesnar ended the brawl with an F-5. The following week on Old School Raw, Mark Henry challenged him again only to have Lesnar dislocate his elbow with the Kimura Lock. Following this, Big Show came out and confronted Lesnar, which lead to a short physical confrontation between the two. Lesnar defeated Big Show at the Royal Rumble. On the February 24, 2014 episode of Raw, Lesnar received a contract to challenge anyone he wanted at WrestleMania XXX in consolation for not being able to compete for the WWE World Heavyweight Championship. The Undertaker returned to confront Lesnar and accepted his challenge for a WrestleMania XXX match. Lesnar would ultimately end Undertaker's streak at the event, extending Undertaker's record to 21 wins and 1 loss.
On the July 21, 2014 edition of Raw, Triple H announced that Lesnar will face John Cena at Summerslam for the WWE World Heavyweight Championship. At Summerslam, Lesnar defeated Cena to become the WWE World Heavyweight Champion. During the match, he delivered 16 German suplexes and two F-5s to Cena. On the August 19 episode of Main Event, Triple H announced that Cena would invoke his championship rematch clause against Lesnar at Night of Champions. Lesnar wound up being disqualified due to Seth Rollins interfering, but Cena thereafter prevented Rollins from cashing in his Money in the Bank contract. Lesnar returned on the December 15 episode of Raw, attacking Chris Jericho who was facing Paul Heyman in a Street Fight. Lesnar took Jericho out with an F5 and then interfered in the steel cage main event between John Cena and Seth Rollins, attacking Cena, helping Rollins to get the win. On the January 12th episode of Raw, Lesnar would be curb-stomped by Seth Rollins during their championship match contract signing that also included John Cena. The following week, Lesnar would open Raw, and called Seth Rollins out, demanding a fight. Later that same night after attempting to get his hands on Rollins, Lesnar would deliver F5s to Kane and Big Show, turning face for the first time since his WWE return, and also the first time since 2003. Lesnar retained the title at the Royal Rumble and it was announced he would defend the title against Rumble winner Roman Reigns at Wrestlemania 31.