Denis Kang's MMA career began in 1998 at the Ultimate Warrior Challenge in Vancouver, Canada. Kang was successful in his debut fight, winning by submission (rear naked choke) over Eric Harcrow. Kang would go on to win his next 2 fights before going 2–5 over his next 8 with one no contest against Dennis Hallman. He would go 7–7 with one no contest in his first 15 fights, with a few convincing defeats.

After winning 5 of his next 6 fights, Kang signed a contract with Spirit MC, a South Korea MMA organization. Denis went 5–0 while fighting in Korea and was crowned the Spirit MC Heavyweight Champion. Following this, Denis relocated to Florida and joined American Top Team.

It was at this time that Kang caught the attention of the PRIDE Fighting Championships. Kang signed a contract with PRIDE and made his debut at PRIDE Bushido 6, where he defeated Japanese opponent Takahiro Oba via armbar at 4:24 of round one. Under the Korean banner, Denis went on to win his next two fights in PRIDE, with a victory coming over Englishman Mark Weir at PRIDE Bushido 10. In between fights for Pride, Denis fought American Albert Basconcelles and knocked him out within 12 seconds of the opening bell. At Bushido 11, Denis Kang continued his winning streak by knocking out Chute Boxe member Murilo "Ninja" Rua in 15 seconds in the first round of the PRIDE Bushido Welterweight (183 lb) Grand Prix. At Bushido 12, Denis submitted Armenian Amar Suloev via rear naked choke to advance to the semi-final round of the Welterweight Grand Prix. With this win, Kang was then 18–0–1 in his last 19 contests. However, after advancing through the semi-final round of the Bushido tournament and beating Akihiro Gono on November 5, 2006 he was defeated by Kazuo Misaki in the final at Bushido 13. Kang had torn his right biceps muscle earlier that night in his win over Gono.

Kang had only one match in 2007 after breaking his hand (an injury that will reappear over and over again) in a match against Jungkyu Choi for the Spirit MC Heavyweight Championship on March 11. Kang won via unanimous decision but was unable to compete for several months.

After the contract termination of PRIDE, Kang moved to K-1 Hero's maintaining the championship of Spirit MC. Kang made his K-1 Hero's debut on October 28, 2007, in "K-1 Hero's in Seoul". His opponent was Yoshihiro Akiyama who was returning to MMA after a 10 month suspension. Kang lost via TKO.

Kang next fight was on April 29, 2008, in the opening round of the DREAM Middleweight Granx Prix, losing to Gegard Mousasi via triangle choke.

On August 30, 2008, Kang defended his Spirit MC Heavyweight Championship by defeating Kim Jae Young by TKO due to strikes at 1:31 of the first round.

Kang defeated UFC veteran Marvin Eastman by TKO on October 25, 2008, at “Raw Combat: Redemption” in Calgary, Alberta, Canada. The fight lasted 48 seconds.

Denis Kang made his UFC debut on January 17, 2009 against Alan Belcher despite suffering from ankle injury. Although Kang controlled the fight, he lost by guillotine choke at the end of round two after a takedown attempt.

Kang redeemed himself with a victory over Xavier Foupa-Pokam at UFC 97 by unanimous decision.

Kang faced Michael Bisping on November 14, 2009 at UFC 105. In an interview about his fight with Bisping, Kang revealed he wanted a title fight with Anderson Silva. Bisping TKO'd Kang in the 2nd round. Even though Bisping was the crowd favourite in his home town Manchester and Kang was being booed on, Kang dropped Bisping with a right hand in the first round which rocked Bisping. Kang followed Bisping to the floor and attempted submissions, but Bisping defended well and neutralised Kang's attacks despite Kang briefly having full mount. In the 2nd round Bisping secured a single-leg takedown and the momentum of the fight shifted : he unleashed a vicious ground and pound attack on the Canadian, which Kang failed to recover from. The fight was awarded Fight of the Night honours, giving both Kang and Bisping a $40,000 bonus check.

Denis Kang was cut from the UFC on Dec. 9 2009, along with Brock Larson, Rolando Delgado and Jason Dent. Kang recently announced he has signed with W1 MMA and will fight for their vacant middleweight title on December 18, 2010 in Montreal, Quebec.

Denis defeated Dae Won Kim at W-1: Judgment day. Kang faced off against former WEC Middleweight Champion Paulo Filho on July 18, 2010 at an Impact Fighting Championships event. The fight was declared a split draw.

Denis joined the Korean MMA promotion Road FC by signing a multi-fight non-exclusive deal.