Sylvia joined a karate school as a child, and began wrestling in high school. After graduation, he worked construction, community gardening, hanging sheet rock, a bouncer for Shane Reids bar, and painting houses. He was playing semi-pro football when became interested in MMA, and began to train with Marcus Davis. During this time he decided to take up boxing and grappling, and after training for a year, Sylvia entered his first grappling contest, taking first place in his division. Soon after, he got the opportunity to fight in a Rhode Island no-holds-barred amateur event, in which he knocked out his opponent in 17 seconds.

A long time UFC fan, Sylvia and some friends attended UFC 28 in Atlantic City, New Jersey in 2000. Arriving early at the arena, they went inside and happened to see former UFC champion and MMA trainer Pat Miletich. Miletich remarked to Tim "You are a big son of a bitch!" and asked "Do you fight?", to which Sylvia replied "A little, but I have a hard time finding training partners and getting fights." Miletich simply said "Come to Iowa", and in late 2000 Sylvia sold all of his belongings and moved to Bettendorf, Iowa, to train with Team Miletich.

Sylvia made his professional MMA debut in 2001 fighting for the IFC. He went on to rack up thirteen consecutive wins with ten knockouts in organizations such as SuperBrawl, and Extreme Challenge. In 2002, Sylvia signed with the Ultimate Fighting Championship and won a TKO victory over Wesley "Cabbage" Correira at UFC 39 when Correira's corner threw in the towel. Sylvia then went on to defeat Ricco Rodriguez at UFC 41, winning his first UFC heavyweight championship. Soon after Sylvia defended his title with another quick victory over Gan McGee at UFC 44.

After the fight with McGee, Sylvia tested positive for the banned substance Stanozolol, an anabolic steroid. He was stripped of his title, handed a six month suspension, and fined $10,000 by the Nevada State Athletic Commission (NSAC). He later commented that the steroid usage was for shedding excess weight. Sylvia apologized and made no effort to claim innocence.

At UFC 48 in June 2004, Sylvia returned to face Frank Mir for the vacant UFC heavyweight championship; he lost. Early on in the fight Mir trapped Sylvia's right arm in an armbar submission attempt. As Sylvia tried to escape the hold, Mir tightened the submission and Sylvia's radius bone snapped about three inches below his elbow. Referee Herb Dean immediately stopped the fight and declared Sylvia unable to continue. Sylvia took exception to the decision and repeatedly claimed his arm was not broken (though the break could be explicitly seen on the slow-motion replay of the fight), even touching it and moving it around to demonstrate. NSAC officials and UFC president Dana White, acting on the doctor's opinion and in the interests of Sylvia's health, officially ended the fight and awarded Mir the title. Sylvia was taken to a local hospital where an x-ray revealed that his arm was in fact broken; he then took several months off to recuperate.

Six months after his loss to Frank Mir, Sylvia returned (a titanium plate had been attached to his forearm) with a victory over Wes Sims at SuperBrawl 38. In February 2005, Sylvia made his return to the UFC, taking on Andrei Arlovski to crown an interim UFC heavyweight champion as Mir, while still the nominal UFC champion, had suffered numerous injuries from a motorcycle accident that left him unable to defend his title. Sylvia was knocked to the ground by an overhand right punch, and while on the ground was caught in an achilles lock. Fearing another injury he tapped out.

In May of 2005, Sylvia defeated Mike Block in the IFC by TKO, then went on to face MMA veteran Tra Telligman at UFC 54. With one second left in the first round, Sylvia landed a left head kick which knocked out Telligman. Sylvia made his cable television debut next, fighting Assuerio Silva at Ultimate Fight Night 3 on Spike TV, winning a unanimous decision after three rounds in a match that was intended to decide the next number one contender.

At UFC 59, Sylvia was finally awarded his rematch with the then-outright champion Andrei Arlovski. During the fight Sylvia was knocked down by Arlovski, only to immediately stand up. Within ten seconds Sylvia dropped Arlovski with an uppercut, followed by ground and pound, securing a TKO victory and once again taking the UFC heavyweight title. Soon after the match, Sylvia requested, and was given, a rematch with Arlovski. Their third match took place at UFC 61, with Sylvia remaining champion by unanimous decision.

Sylvia next fought Jeff Monson at UFC 65. Billed as a classic "striker vs. grappler" match, Sylvia won again by unanimous decision. The fight showcased Sylvia's growth as a mixed martial artist as he avoided danger from Monson on the ground, and even attempted a triangle choke submission himself.

On March 3, 2007, Sylvia lost the UFC heavyweight championship to Randy Couture via unanimous decision. The judges all scored the bout 50-45, giving Couture his fifth UFC championship.

On October 20, 2007 at UFC 77, Sylvia defeated Brandon Vera via unanimous decision. In the after fight interview he called out Cheick Kongo to fight for the spot as number one contender for the heavyweight championship belt.

On February 2, 2008 Sylvia lost via guillotine choke to Antonio Rodrigo Nogueira at UFC 81 in Las Vegas, Nevada for the interim heavyweight championship.