He competed as a lightweight on The Ultimate Fighter 5 television series. During tryouts and production for the show, he told the UFC that he was 4–0, but he was actually 2–0, both wins from amateur bouts. His first professional fight (albeit technically an exhibition) was on the show, a victory against Rob Emerson which he won by a very controversial unanimous decision after three rounds. After defeating Rob Emerson by decision in the preliminaries of TUF 5, Hill went on to lose in the quarterfinals via triangle choke in the first round to fellow Team Pulver teammate, and future TUF 5 champion, Nate Diaz. Although he lost, Hill’s performance validated the comments made several weeks prior by guest coach Jeremy Horn, who suggested Hill had the potential to be a dominating force in the UFC Lightweight division—even going so far as to offer to train Hill after his run on the show.
In his UFC debut at UFC Fight Night 12, Corey Hill defeated Joe Veres with a second round TKO. Hill followed up with an appearance at UFC 86 against former ICON Sport Lightweight champion Justin Buchholz, who submitted him via rear-naked choke in the second round.
During the UFC: Fight for the Troops event in December 2008, Hill fractured his right shin while throwing a kick that was checked by Dale Hartt. Recovery for the injury was expected to take 12–18 months. With regard to organizational support, Hill told Kevin Iole of Yahoo! Sports, “The UFC was awesome throughout this whole thing and was so good to me, I feel indebted to them.” An XCF event helped to raise money for the injured UFC fighter.