Early career Davis entered professional MMA on August 22, 2003, with a first-round TKO of Shaun Gay. Though later describing himself as "one-dimensional", Davis was able to put together a 3–2 professional record.
Ultimate Fighting Championship In 2005, Marcus was a contestant on
The Ultimate Fighter 2. Coming in with reputed boxing skills, he picked
Joe Stevenson in episode four in what proved to be a mismatch as Davis was quickly taken down by the eventual winner and submitted. He was later brought back in episode ten to cover for
Jason Von Flue, who suffered a cut on his forehead in practice, though Von Flue's cut was healed enough in time to fight. Marcus fought on the undercard of the finale, where he lost to
Melvin Guillard. Following the series, Davis strongly considered retirement but recuperated from a shoulder injury picked up against Joe Stevenson and went back to MMA training. Davis subsequently bounced back with 11 wins, including a triumphant return to the UFC at
Ortiz vs. Shamrock 3: The Final Chapter. In Davis' fight against
Paul Taylor at
UFC 75 in London, England he was knocked down by a high kick to the neck, but was able to recover, gain a dominant position, and secure an armbar as Taylor desperately scrambled to regain control. Marcus won Fight of the night as well as submission of the night for that fight. For earning the submission and fight of the night awards, he was awarded $80,000 in bonuses. His next fight was a highlight-reel knockout win against fierce rival
Jess Liaudin at
UFC 80 in
Newcastle, England. Marcus Davis lost a very one sided unanimous decision to
Mike Swick at
UFC 85, ending his six fight undefeated streak in the octagon and eleven fight winning streak in MMA. He got back to winning ways with a submission victory over England's
Paul Kelly at
UFC 89. Davis' fought
Chris Lytle at
UFC 93 on January 17, 2009. Both fighters are former boxers and had discussed a potential fight in their futures since early 2008. Lytle called out Davis at
UFC 89. He suggested that the fight take place in Ireland, where Davis had gained a substantial following due to his emphasis on his Irish roots. Leading up to the event, both fighters promised to stand and trade blows for the entire fight, rather than engage in grappling. Lytle managed to stun Davis several times, but Davis used superior footwork, counter-punching, and kicks to win a split decision. The bout shared Fight of the Night honors with the
Coleman/
Rua co-main event, earning Davis a $40,000 bonus. Davis fought on June 13, 2009, against up and coming striker
Dan Hardy. The fight became personal when Hardy took offense to Davis defeating British-based fighters in Britain. Hardy called Davis a "fake Irishman" as well as a "
Plastic Paddy" and said his website "looked like a St. Patrick's day parade had blown up". Hardy posted photos of Davis on Cagewarriors.com which resulted in many being edited in "humorous" fashions by forum users. Some of these edits included homosexual imagery which touched a nerve with Davis and increased the ill-feeling between the two. Hardy won the fight via a narrow and controversial split decision. After the fight, Davis expressed his anger at the decision, stating, "I won the fight. I won 12 minutes of the fight and they gave it to him? You bet I want a rematch, right away." Davis suffered his first knock out loss by
Ben Saunders due to knee on November 21, 2009, at
UFC 106. This knockout loss resulting in Davis receiving a medical suspension of up to six months because of a broken nose. Davis fought
Jonathan Goulet at
UFC 113. He won via TKO due to punches in the second round. Davis lost to
Nate Diaz on August 28, 2010, at
UFC 118 via technical submission (guillotine choke) in the third round. In an interview with MMA Junkie.com Davis confirmed he would drop to the Lightweight division for his next UFC appearance and said he made the decision after the realization that his frame is better suited for 155 pounds. Davis also confirmed that he was contemplating retirement stating "I'm going to finish out my career at 155 and try to entertain some people along the way before my old ass retires," Davis also stated "That's where I'm normally and naturally going to be, so that's where I'm going to fight right now," he said of the 155-pound lightweight division. "It does make more sense that I should be able to compete better at that weight for the fact that the weight difference isn't going to be so dramatic." Davis fought
Jeremy Stephens on January 1, 2011, at
UFC 125. After a back and forth fight which saw Davis getting an edge on the judges' scorecards, he was knocked out by a counter right hook at 2:33 of the third round. On January 6, 2011 it was announced that Davis had been released by the UFC after losing four out of his last five bouts.
Maximum Fighting Championship It was announced that Davis signed a contract with the
Maximum Fighting Championship company, and had signed on to fight Curtis Demarce at
MFC 29. He won the fight via split decision. Davis returned at
MFC 30, defeating
Pete Spratt by unanimous decision in a rematch of their bout from
UFC 69.
Independent promotions Davis was scheduled to face Josh Hersey on September 23 at Global Fight League 13: Heavy Hitters in
Portland, Maine, but Hersey pulled out of the bout due to a knee injury and was replaced by Travis Coyle. However the event was postponed. On May 6, 2012, Davis turned his hand to
kickboxing to challenge Mark Casserly for the
ISKA World Light Cruiserweight (-84.6 kg/186.5 lb)
Oriental Championship in Dublin, Ireland, losing by head-kick knockout in round one. Davis replaced
Karo Parisyan and faced
David Bielkheden at Superior Challenge 8 on October 6, 2012, in Malmo, Sweden. He won by unanimous decision. Davis fought against Darrius Heyliger on May 18, 2013 at NEF 7. Davis defeated Heyliger via unanimous decision. Following his loss to
Alexander Sarnavskiy, Davis faced Ryan Sanders at NEF: Fight Night 12 on February 8, 2014. In a shocking upset, Davis lost the fight via doctor stoppage TKO.
Bellator MMA Davis made his
Bellator debut on March 21, 2013, at
Bellator 93 against
Waachiim Spiritwolf. The bout ended in a no contest when Davis hit Spiritwolf with an inadvertent knee to the groin in the first round and he could not continue. Davis faced
Alexander Sarnavskiy on September 27, 2013, at
Bellator 101 in the Quarterfinals of
Bellator's Season Nine Lightweight Tournament. He lost via rear-naked choke submission in the first round. A comment was made via Marcus Davis' YouTube account that he would be retiring in 2014.
Return to competition On July 21, 2020, news surfaced that Davis is expected to face Nick Alley at Premier FC 31 on October 3, 2020. The bout was later cancelled and Davis was rebooked against Stephen Stengel on November 19, 2021 at Premier FC 32. He won the fight via first-round submission. ==Other media==