In June, 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 in the main event at Summerslam, where he defeated The Rock. Lesnar, at age 25, was then the youngest WWE Champion ever (as of 2005, he is still technically the youngest WWE Champion ever, but Randy Orton is the youngest world champion in WWE history, having won the World Heavyweight Championship at 24). The next night on Raw, SmackDown! General Manager Stephanie McMahon announced Lesnar would defend the title only on Smackdown, forcing Raw GM Eric Bischoff to create the World Heavyweight Championship. That fall, Lesnar feuded with the Undertaker, who he defeated in a Hell in a Cell match at No Mercy 2002. He lost the championship in November at the Survivor Series pay-per-view to The Big Show in under six minutes after being betrayed by Paul Heyman.
After winning the 2003 Royal Rumble match in January 2003, Lesnar regained the WWE championship in the main event of WrestleMania XIX from Kurt Angle. Toward the end of the match, his attempt at the Shooting Star Press failed, as Angle was too far from the ring post. Despite the fact that Lesnar had made that distance in the past with his Shooting Star Press attempts in OVW this time he was exhausted and couldn't make the complete distance. Lesnar landed correctly for the move, but short of his mark (on top of Angle's chest). The result was that Brock's head slammed into the side and ribcage of Kurt Angle at the end of his rotation out of the SSP. This stunned Brock and forced Angle to improvise the finish of the match. The botched move gave Lesnar a concussion. This was Angle's last match in WWE for several months, as he took some time off to have neck surgery. Later Brock admitted in interviews that this event took away some of his confidence and led to him realizing that he could get seriously hurt in the ring. This realization made him second guess his vocation and eventually helped in his decision to briefly leave Professional Wrestling and try out for the NFL (particularly the Minnesota Vikings; see "Departure from WWE" section below).
When Angle returned he befriended Lesnar, but when Lesnar lost the title back to Angle in a triple threat match with the Big Show at Vengeance in July, Lesnar turned on him and developed a mean streak, which was highly supported by WWE owner Vince McMahon. In August on Smackdown, Lesnar faced amputee wrestler Zach Gowen, who had an artificial leg due to cancer, in Gowen's hometown. Lesnar physically broke Gowen's remaining leg in front of his little brother and mother. Later, Lesnar kidnapped a wheelchaired Gowen and pushed him off some steps. Lesnar regained the WWE Title from Kurt Angle in an Ironman match. He would later have a brief feud with Hardcore Holly. In this storyline, Holly got some revenge from Lesnar for breaking his neck. Holly told him that he was going to break Lesnar's own neck. The feud however ended when Lesnar defeated Holly to retain the WWE Championship at the Royal Rumble. In February he lost the title to Eddie Guerrero at the No Way Out event.
Before losing the WWE Championship to Guerrero, Lesnar had begun a feud with RAW's Goldberg. The feud began at Survivor Series 2003, while both men were champions of their respective brands. Lesnar would attack Goldberg at Royal Rumble 2004, and Goldberg would retaliate at No Way Out in Lesnar's match against Guerrero. This set up an interbrand match for WrestleMania XX between the two.
Lesnar, who was angered by RAW's "Stone Cold" Steve Austin having suggested Goldberg attack Lesnar at No Way Out, then attacked Austin, and stole his four-wheeler. Austin was then inserted as the Special Guest Referee for the match. Although it was already known that the match would be Goldberg's last in WWE, it was only a week before WrestleMania that rumors surfaced Lesnar too was leaving, in order to pursue a career in the NFL. As a result, the fans at Madison Square Garden jeered both wrestlers with chants of "you sold out" & "Na Na Na Na Hey Hey Hey Good Bye". Lesnar was said to be particularly shaken by this reaction. Because both men were leaving WWE, it was clear that the only man who would go over in the match would be Austin. Sure enough, although Goldberg won the match, both men were given Stunners by Austin on their way out.