Ultimate Fighting Championship

Clementi began his professional career in 1999 and amassed a 13–6 record before making his Ultimate Fighting Championship (UFC) debut at UFC 41, where he lost by submission to Yves Edwards. Clementi continued to fight in smaller promotions before appearing as a contestant on The Ultimate Fighter 4 reality show, which focused on promising fighters that had not succeeded in their UFC careers. With the namesake of "Team No Love" Clementi lost his preliminary bout against Shonie Carter by unanimous decision. He appeared on the undercard of the season finale as a lightweight, but lost by rear naked choke submission to Din Thomas.

Despite his losses, Clementi continued to appear in the UFC. He secured his first victory in the promotion via rear naked choke submission against Ross Pointon at UFC Fight Night 8, although the fight was not aired during the broadcast. After losing to Roan Carneiro at UFC Fight Night 9, Clementi alternated between appearing in the UFC and smaller promotions. He took a fight with Anthony Johnson at UFC 76 on two weeks' notice and won by rear naked choke submission. With the win, Clementi improved his official UFC record to 2–3.

Clementi followed up with a win at UFC 79 against Melvin Guillard. Guillard tapped from a rear naked choke. Prior to the match, Clementi and Guillard engaged in a war of words.

In the span of a month, Clementi racked up two more victories against Sam Stout and Terry Etim at UFC 83 and UFC 84 respectively. By this time, Clementi was riding a four-win streak in the UFC and a six-win streak in his career. The streak ended at UFC 90, where he lost to Gray Maynard by unanimous decision. Clementi followed up with an appearance at UFC Fight Night: Lauzon vs. Stephens, where he lost to Gleison Tibau by guillotine choke. With his second consecutive loss in the UFC, Clementi was cut from the promotion's roster.^[2]

Independent Promotions

On June 27, 2009, Clementi was in the main event for the first ever MMA fight in Pennsylvania. The promotion company UCFC held its "Rumble on the Rivers" event in Pittsburgh's Mellon Arena. Clementi was unable to continue the fight after only 40 seconds into the first round because of a broken collar bone after a takedown and the ringside doctor stopped the fight.

On November 29, 2009, Clementi headlined the event "Cage Wars: Nightmare" in Belfast, Northern Ireland and fought local prospect Chris Stringer, defeating him by triangle choke in the second round, after dominating the fight.^[3] Clementi followed that win by headlining the Team 3:16 – Season's Beatings Event in Shreveport, Louisiana, defeating Derrick Krantz in the first round via armbar.

On January 16 at the 5150 Fight League's "The New Year's Revolution" show Clementi defeated WEC veteran Mike Budnik via rear naked choke to win the 5150 Combat League Lightweight Championship and improving his record upon being cut from the UFC to 4–1.

Bellator Fighting Championships

Clementi was signed by Bellator and made his debut at Bellator 28 against Carey Vanier in a Bellator Lightweight tournament Qualifying bout, He lost by split decision.

On May 25, 2012, Clementi returned to Bellator at Bellator 70, he defeated Derek Campos by guillotine choke in round one.

Clementi faced Alexander Sarnavskiy on October 19, 2012 at Bellator 77 in a Lightweight Tournament Quarter Final bout.^[4] He won the fight by split decision.

Clementi faced Marcin Held in the Semifinals on November 16, 2012 at Bellator 81 and lost via toe hold submission in the second round.^[5]

On November 19, 2012 Clementi announced his retirement after competing in MMA for 13 years.^[6]

On July 24, 2013, Clementi announced he has come out of retirement and returned to Bellator to complete in the promotions Season Nine Lightweight Tournament, He was set to face UK Fighter Rob Sinclair on September 27, 2013 at Bellator 101 in the Quarterfinals.^[7] But on September 10, 2013, it was announced that Sinclair had to pull out of the fight due to a knee injury and was replaced by Ricardo Tirloni.^[8] He lost via unanimous decision.