Ultimate Fighting Championship

Carwin made his UFC debut on the undercard of UFC 84 against Christian Wellisch. He won the match via one punch knockout 44 seconds into the first round, demonstrating his punching power by sending Wellisch's mouthpiece flying across the octagon. Carwin followed up with an appearance in Birmingham, England at UFC 89 against Neil Wain, winning another one sided fight via TKO at 1:31 of the first round. At UFC 96, Carwin took a major step up in competition when he faced former number one contender and renowned Brazilian jiu-jitsu black belt Gabriel Gonzaga. After being stunned by a flurry of punches, Carwin knocked Gonzaga unconscious with a short right hand at 1:09 of the first round.

Carwin was then set to face fellow undefeated heavyweight contender Cain Velasquez at UFC 104, with the winner getting a title shot against then-champion Brock Lesnar;^[6] however, the UFC reconsidered the bout and Carwin was confirmed to fight for the title at UFC 106 on November 21, 2009.^[7] The fight was later postponed after Lesnar pulled out due to illness, with UFC 108 a likely date. Lesnar then pulled out of that fight on October 26, and Dana White reported at the UFC 105 post-fight press conference that Lesnar was too ill to compete^[8] and was expected to be out for the first half of 2010 while recovering from an intestinal disorder and subsequent surgery.^[9][10]

Carwin fought former UFC Heavyweight Champion Frank Mir for the UFC Interim Heavyweight Championship on March 27, 2010, at UFC 111.^[11] Carwin stated on his website that, "Mir is a legend of the sport, and I would be honored to fight him."^[12] Early in the first round, Carwin dropped Mir with multiple uppercuts from the clinch. Carwin then followed Mir to the ground, took back mount and rained down heavy punches on Mir's head thus winning the fight via knockout to become the UFC Interim Heavyweight Champion. The bout also marked the longest fight of Carwin's career at three minutes and 48 seconds; his previous mark was his MMA debut against Carlton Jones, with a time of 2 minutes and 11 seconds.^[13] Following UFC 111, Beau Dure of USA Today compared Carwin to Mike Tyson,^[14] echoing a similar comparison made prior to the event by Mark Wayne of Fightline.com.^[15]

Carwin then went on to face Brock Lesnar for the undisputed UFC Heavyweight Championship at UFC 116. He dominated the first round, knocking Lesnar down and unleashing a relentless ground and pound attack to a visibly stunned Lesnar. However, in the second round, Lesnar took an exhausted Carwin down and applied an arm-triangle choke, forcing Carwin to submit.

Carwin was scheduled to face former IFL Heavyweight Champion Roy Nelson on January 1, 2011, at UFC 125;^[16][17] however, he later announced on his website that he would pull out of the fight due to back and neck pain, which required surgery that took place on November 2, 2010. Carwin later stated that the neck surgery was a success.^[18] On January 3, 2011, Carwin posted a blog entry on his website, stating that he had just begun his first day of training since his neck surgery, hoping to be back in the octagon by May or June 2011.^[19]

Carwin was expected to face promotional newcomer Jon Olav Einemo on June 11, 2011, at UFC 131.^[20] However, another bout with diverticulitis sidelined Brock Lesnar, so Carwin agreed to step in to the main event to fight Junior dos Santos.^[21] Carwin was defeated by Dos Santos via unanimous decision (30–27, 30–27, and 30–26). This fight was the first in Carwin's career to ever go to a decision.

On September 9, 2011, Carwin posted a blog entry on his website stating that he was 100% healthy and looking to return to the octagon in late December or early January.^[22] On October 16, 2011, it was revealed that Carwin would need back surgery. He was expected to be out until mid-2012.^[23] However, his recovery took longer than expected, and he was targeting a return to fighting that fall.^[24]

On Thursday July 12, 2012, Dana White announced that Carwin and Roy Nelson had been picked as the next coaches for the 16th season of The Ultimate Fighter (TUF) and were expected to face each other on December 15, 2012, at The Ultimate Fighter: Team Carwin vs. Team Nelson Finale.^[25] However, on November 14, 2012, Dana White revealed on Twitter that Carwin injured his knee and would not be facing Nelson in the TUF finale.^[26]

On May 7, 2013, Carwin announced his retirement from mixed martial arts (MMA) after enduring several injuries since his last fight on June 11, 2011.^[27]

MMA return

On July 26, 2016, Carwin announced his plans to return to MMA, five years since his last competitive bout.^[28] On September 7, 2016, Carwin announced that he had worked with his management team at Ingrained Media to secure his release from the UFC and became a free agent.^[29] On October 28, 2016, Carwin announced he had signed with Rizin Fighting Federation. He stated that he expected to debut on their December 29, 2016, card, but later withdrew with injury.^[30]

In late 2017, Carwin was in negotiations with Bellator MMA but nothing came to fruition, despite his injuries being rehabilitated.^[31]

Mixed martial arts

• Ultimate Fighting Championship
  • Interim UFC Heavyweight Championship (One time)
  • Knockout of the Night (One time) vs. Frank Mir
• Ring of Fire
  • ROF Heavyweight Championship (One time)

Amateur wrestling

• National Collegiate Athletic Association
  • NCAA Division II Wrestling Hall of Fame (2011)^[36]
  • NCAA Division II Wrestling Heavyweight Champion (1999)
  • NCAA Division II National Wrestling Heavyweight Runner-Up (1996, 1997)
  • NCAA Division II All American (1996, 1997, 1999)
• Western State College
  • Western State College Mountaineer Sports Hall of Fame (2004)
• Rocky Mountain Athletic Conference
  • Rocky Mountain Athletic Conference Hall of Fame (2010)