Foster began his professional MMA career in 2006, after his brother died. In an interview with FightLockdown.com, Foster stated that "martial arts have changed me, it's made me a more humble and respectful individual. My brother’s death did make me a very angry person, so very shortly after it happened, I started training and used the physical stress of training to cancel out the emotional pain caused by his death. I like to think he is very proud of what I have accomplished since his passing I take a photo of my brother with me to every fight."
Foster got his start fighting for the Masters of the Cage company. Foster fought six times for the promotion and went 4–2 and won eight consecutive fights before losing to future The Ultimate Fighter winner, Diego Brandão. Foster bounced back and won his next three fights before getting signed by the Ultimate Fighting Championship.
At UFC 103, Foster was set to make his debut against highly regarded Englishman, Paul Daley. Daley replaced Mike Swick in a fight with Martin Kampmann, leaving Foster without an opponent. Instead of Daley, he ended up fighting Rick Story and lost via arm-triangle choke submission in the second round. At UFC 106 on November 21, 2009, Foster defeated Brock Larson by TKO in the second round after taking an illegal kick to the head and an illegal knee to the head in the first round. Foster stepped in for an injured Dong Hyun Kim at UFC 110 to face Chris Lytle. He lost due to a kneebar submission in the first round. Foster next faced Forrest Petz on September 15, 2010 at UFC Fight Night 22. He won in the first round by TKO. Foster then faced Matt Brown at UFC 123, replacing an injured Rory MacDonald. Foster won the fight by guillotine choke submission in the second round. In 2010, an inadvertent groin strike sustained during training burst Foster's testicle and it had to be surgically removed. Later, when he was cleared to continue training for upcoming Pierson fight at UFC 129, a pre-fight MRI showed clear signs of a brain hemorrhage. The second MRI taken of his brain showed no signs of hemorrhage. However, Foster was left without a fight and subsequently he was released from UFC. According to Foster, there was miscommunication in the media leading to reports of him going through a brain surgery but Foster stated they were false.
Post-UFC
Now fully recovered, Foster's next fight was against Jack Mason at Cage Warriors Fighting Championship 44. He won the fight by standing guillotine choke submission in the first round. Foster defeated LaVerne Clark via first-round submission at Capital City Cage Wars 7 on October 15, 2011.
World Series of Fighting/Professional Fighters League
Foster joined the World Series of Fighting in 2014. In his first fight for the promotion, Foster faced Jake Shields in the main event of WSOF 17 on January 17, 2015. He lost the fight via submission in the first round. In his second fight for the promotion, Foster defeated LaRue Burley via knockout with a right hook at 34 seconds of the first round at WSOF 23 on September 18, 2015. Foster then entered WSOF's one night Lightweight tournament to determine the number one contender for the Lightweight Championship. He faced Joáo Zeferino in the quarter finals and lost via submission due to a heel hook. However, a number of injuries plagued the semifinalists and Foster continued as an injury substitution. He faced Luis Palomino in the semifinals and won via TKO in the second round. In the finals, he faced Zeferino in a rematch and won the fight via knockout in the second round to win the tournament. Foster faced Luiz Firmino at on October 7, 2016. He won the fight via triangle choke submission in the first round. Foster faced Jon Fitch in a welterweight title bout on June 30, 2017 at Professional Fighters League 36: Fitch vs. Foster. He lost the fight in the second round due to submission. Foster faced Ramsey Nijem in the initial round of Professional Fighters League's lightweight tournament. Foster won the fight via ground and pound, stemming from a flying knee to Nijem's head.
