After compiling an amateur MMA record of 16-0, Kimmons made his professional mixed martial arts debut on November 21, 2003 for the HOOKnSHOOT promotion. He knocked outDavid Webster in the first round to notch his first professional win. Rob won two more times in 2004 and was signed to World Extreme Cagefighting and was given a fight against Joe Riggs for the vacant middleweight title. Kimmons lost the fight in the first round. After winning six straight fights since the loss to Riggs, Kimmons took his second loss this time to Ryan Jensen. Kimmons fought for the World Series of Rumble Welterweight title in 2006 and defeated Steve Schneider to become champion. He then fought in Shooto's American Welterweight Grand Prix where he defeated all competition with first-round knockouts or submissions to win the 16 man tournament. In July 2007, Kimmons faced UFC veteran, Marvin Eastman for the International Fighting Organization's middleweight title. Kimmons lost the fight by decision. Kimmons then dropped down to the welterweight division two months later in order to face Tristan Yunker for the IFO's welterweight title. Kimmons won the title in the first round.
Kimmons made his debut defeating Rob Yundt on the undercard of TUF 7 Finale. Kimmons submitted Yundt with a guillotine choke in round 1. He had his second fight for the company on the UFC Fight Night 15 card taking on Dan Miller. Miller dominated the fight and submitted Kimmons early in round one. Kimmons bounced back with a win on the UFC Fight Night 18 card by choking Joe Vedepo unconscious. Kimmons won submission of the night honors. After suffering a TKO loss to Jorge Rivera, Kimmons plans to move to the UFC welterweight division instead of middleweight. Kimmons was scheduled to face Mike Pierce on March 21, 2010 at, but Kimmons was forced off the card with an injury. Pierce is now set to face UFC newcomer Julio Paulino. Kimmons then faced Steve Steinbeiss on August 1, 2010 at UFC on Versus 2 and won via unanimous decision. Kimmons lost to Kyle Noke on November 13, 2010 at UFC 122 via submission at 1:33 of round 2. Kimmons next faced Dongi Yang on March 3, 2011 at. He lost the fight via TKO due to punches near the end of the second round and was subsequently released from the promotion.
Post UFC
He faced Chuck Parmelee on September, 10th 2011 winning the fight in 53 seconds via Guillotine Choke. He faced Andre Kase on October, 6th 2012 winning the fight 3:14 in the first round via TKO due to Knees. He faced Dan McGlasson on November, 17th 2012 winning the fight via Submission due to strikes.
