Smith would defend his championship from Justin Levens at WEC 18 before scheduling a fight with the Ultimate Fighting Championship in April 2006, moving down a weight class to fight David Terrell at UFC 59 . The fight marked with questionable officiating ended in controversial fashion, prompting Smith to file a complaint with the California State Athletic Commission.

In the early half of his career, Smith - fighting at heavyweight - compiled victories in the International Fighting Championships and Gladiator Challenge shows across the United States. While successful in the smaller promotions, Smith's first major appearance in the mixed martial arts world came in the World Extreme Cagefighting organization. Smith entered a WEC light heavyweight tournament with limited name recognition, but was branded by experts as the dark horse of the competition.

Smith defeated WEC veteran Tim McKenzie in the semi-finals. With that victory, Smith was slated to fight Justin Levens in the tournament finals; Levens, however, was halted on doctor's advice due to a shoulder injury he sustained in his semi-final bout. The tournament alternate, Tait Fletcher, was brought into the championship round instead. Smith made short work of Fletcher, scoring a first-round technical knockout to become the WEC Light Heavyweight Champion.

Smith would defend his championship from Justin Levens at WEC 18 before scheduling a fight with the Ultimate Fighting Championship in April 2006, moving down a weight class to fight David Terrell at UFC 59. The fight - marked with questionable officiating - ended in controversial fashion, prompting Smith to file a complaint with the California State Athletic Commission.

Smith was then seen as a middleweight contestant on The Ultimate Fighter: The Comeback, where he quickly lost his quarterfinal matchup against Travis Lutter. Smith appeared on the season finale, knocking out Pete Sell in the second round of a very exciting fight in a sequence that saw Smith sustain a hard punch to the liver, and throw the bout-winning knockout punch before falling to the ground in pain.

Smith lost his next fight in the UFC by decision against Patrick Cote at UFC 67. More recently, Smith defeated Troy Miller by technical knockout at Palace Fighting Championship 2 in Lemoore, California. He lost against Ed Herman at UFC 72. Herman submitted Smith with a rear naked choke at 2:25 in the second round after Herman landed a nasty elbow on Smith in the first round covering his face in blood.

Smith made his EliteXC debut on February 16th knocking out Kyle Noke seven seconds into round two. The victory earned him a title shot against Robbie Lawler. The bout was declared a no contest after an accidental poke in the eye in the third round. The first two rounds were so exciting that Gary Shaw awarded both fighters their win bonuses and announced an immediate rematch. Smith lost the subsequent rematch to Lawler via TKO on July 26, 2008 at EliteXC: Unfinished Business. At Strikeforce: Destruction, Smith took a fight with Terry Martin at one week notice and beat Martin in the 1st round by KO.