Brock Lesnar made his WWE debut the night after WrestleMania X8 at the side of Paul Heyman and he was quickly billed as "The Next Big Thing." In keeping with his moniker, he won the WWE Championship quicker than any man in WWE history. After winnning King of the Ring in June 2002, he defeated The Rock for the title at SummerSlam in August 2002, and then defeated Undertaker in Hell in a Cell before dropping the title to Big Show at Survivor Series when Heyman double-crossed him. Lesnar cost Big Show the title in a match against Kurt Angle but it turned out Heyman was also aligned with Angle. This confusion culminated at WrestleMania XIX when Lesnar beat Angle for the title. Angle beat Lesnar and Big Show in a Triple Threat match to regain the title in July 2003, and he beat Lesnar in a singles match at SummerSlam 2003, but he lost the belt to Lesnar in an 60-minute Iron Man Rematch. Lesnar came face-to-face with Goldberg at Survivor Series 2003, which led to a physical altercation between the two at the Royal Rumble in January 2004. Goldberg cost Lesnar the championship in February 2004, and they had a dream match at WrestleMania XX, which turned into a nightmare because both men were leaving the company.
Brock Lesnar left WWE to pursue his luck in the NFL, where he was a walk-on free agent with the Minnesota Vikings, but he did not make the team. Unfortunately, he signed an extensive no-compete contract with WWE in order for his release, which prevented him from working in other avenues of sports entertainment for 10-years. His lawyers argued that such a contract was invalid because it was unreasonable. WWE's lawyers argued that Lesnar understood fully the magnitude of the contract when he signed it. The case was settled before it went to court, and obviously, WWE got a percent of his earnings for his first UFC matches because they advertised those events on their website and during their programming (additionally, footage of his matches were shown in UFC advertisements for the event).
Lesnar was successful in UFC, defeating Randy Couture for the UFC Championship in November 2008. Although most of his title reign was spent on the shelf recovering from both mononucleosis and diverticulitis, he successfully defended against Frank Mir in July 2009 and Shane Carwin on July 3, 2010, in one of the best UFC events of all-time. He lost the title in short fashion to Cain Velasquez in October 2010. His rematch never materialized as Lesnar underwent another surgery on May 27, 2011. Eventually, Lesnar returned to the UFC octagon again in December 2011, losing to Alistair Overeem and then announcing his retirement.
After his UFC Championship loss in October 2010, Brock Lesnar was confronted by Undertaker, which the dirtsheets and Rumor Mills insisted was allegedly a challenge to face him at the up-coming WrestleMania XXVII. Nothing came of it and the event was mostly forgotten until he appeared on the roster for WWE'12. Then, Lesnar returned to WWE-TV the night after WrestleMania XXVIII to flatten John Cena.
Brock Lesnar released an autobiography in 2011, co-written by Paul Heyman who continued a working arrangement with him in UFC, not unlike his managerial relationship with Lesnar through WWE in 2002.