Riley made his professional debut in 1997 at the age of 16 for the HOOKnSHOOT organization. He received his first career loss in his second fight via kneebar, and then won ten consecutive fights. Riley then lost two consecutive fights against future UFC veterans Yves Edwards and Falaniko Vitale.
Riley made his PRIDE debut at PRIDE Bushido 7 against Michihiro Omigawa, who was making his mixed martial arts debut. Riley was successful in his first and only fight with the Japanese organization, winning via head kick knockout in the first round. Riley then participated in two bouts for the BodogFIGHT organization. His first fight was in Costa Rica, which he won via armbar and then fought against future Bellator Lightweight ChampionEddie Alvarez, but lost via knockout. In Riley's next fight, he debuted in the International Fight League against Ryan Schultz, who was the last IFL Lightweight Champion. Riley lost the fight via unanimous decision.
Return to the UFC
Riley returned to the UFC on November 15, 2008 at UFC 91 against Jorge Gurgel. Aaron beat Gurgel by unanimous decision, winning Fight of the Night honors. Riley next faced TUF 8 cast memberShane Nelson at UFC 96, losing via controversial first round TKO. A rematch with Nelson was scheduled immediately, for UFC 101, which Riley won via unanimous decision. At UFC 105 he was defeated by TUF winner Ross Pearson via TKO in the second round after a flying knee by Pearson opened up several cuts on Riley's face. Doctors viewed the cuts on Riley's face, and came to the conclusion that he was unable to continue. Riley then rebounded with a unanimous decision win over Joe Brammer on May 29, 2010 at UFC 114. Riley was expected to face UFC newcomer Pat Audinwood on September 25, 2010 at UFC 119, but Riley was forced from the card with an injury and replaced by Thiago Tavares. Riley was defeated by Tony Ferguson on September 24, 2011 at UFC 135. While the fight was competitive early, Ferguson landed a strong left uppercut, and Ferguson dominated the remaining two minutes of the round. Immediately after the end of the first round, Riley told his corner that his jaw was broken. The fight was declared a TKO victory for Ferguson, as ringside doctors would not allow Riley to continue. Riley had previously suffered a broken jaw when he fought Spencer Fisher at UFC Fight Night 3. Riley was expected to face Cody McKenzie on May 15, 2012 at. However, Riley was pulled from the event and replaced by promotional newcomer Marcus LeVesseur. After nearly two years away from the sport, Riley returned to face Justin Salas on July 27, 2013 at UFC on Fox 8. He lost the back-and-forth fight by split decision. He retired from MMA following his loss to Salas.