Kelleher made his promotional debut against Iuri Alcântara on short notice at UFC 212 on June 3, 2017. He won the fight via submission due to a guillotine choke in the first round and was awarded a Performance of the Night bonus. He next faced Marlon Vera on July 22, 2017 at UFC on Fox 25. He lost via armbar submission in the first round. Kelleher faced Damian Stasiak on October 21, 2017 at UFC Fight Night 118. He won the fight via technical knockout in the third round. This win earned him the Fight of the Night award. Kelleher faced Renan Barão on February 24, 2018 at UFC on Fox 28. He won the fight by unanimous decision. Kelleher faced John Lineker on May 12, 2018 at UFC 224. He lost the fight via knockout in the third round. Kelleher was originally scheduled to face Domingo Pilarte on November 3, 2018 at UFC 230. Pilarte was forced to withdraw from the bout, citing injury, and Kelleher is now slated to face Montel Jackson. At the weigh-ins, Kelleher weight at 137 pounds, 1 pound over the bantamweight non-title fight limit of 136. He was fined 20 percent of his purse, which went to his opponent Jackson. On November 3, it was reported that Kelleher withdrew from the bout due to illness and thus the fight was cancelled. The pairing was left intact and rescheduled for December 29, 2018 at UFC 232. At the weigh-ins, Jackson weighed in at 137 lbs, 1 pound over the non-title fight bantamweight limit of 136 lbs. Jackson was fined 20 percent of his purse to Kelleher and the fight proceeded at catchweight. Kelleher lost the fight via submission in the first round. Kelleher was scheduled to face Mitch Gagnon on May 4, 2019 at UFC Fight Night 150. However, Kelleher pulled out of the fight on April 10 citing injury and was replaced by promotional newcomer Cole Smith. As the last fight of his prevailing contract, Kelleher faced Ode Osbourne at UFC 246 on January 18, 2020. He won the fight via a submission in the first round. This win earned him a Performance of the Night award. After negotiating a new contract, Kelleher faced Hunter Azure in a featherweight contest on May 13, 2020 at. He won the fight via knockout in round two. Kelleher faced Cody Stamann on June 6, 2020 at UFC 250. He lost the bout via unanimous decision.
