Pierson was set to compete in Bellator's Second Season Welterweight Tournament but an injury forced him out of the tournament. He was set to have his first fight against Dan Hornbuckle which would have taken place at Bellator 15.
After TJ Waldburger had to pull out of his fight with Matthew Riddle at UFC 124 due to injury, it was announced on October 27 that Pierson had signed with the UFC to step in on short notice. After outstriking Riddle over three fast-paced rounds, Pierson won a unanimous decision. The fight was dubbed by Dana White as the real Fight of the Night. Pierson was expected to face Brian Foster on April 30, 2011 at UFC 129. However, Foster was forced out of the bout with an injury, and replaced by Jake Ellenberger. Pierson lost to Ellenberger via first-round KO after he was caught by a counter left. Pierson fought Dong Hyun Kim on December 30, 2011 at UFC 141. Pierson was defeated by Kim via unanimous decision. Pierson defeated Jake Hecht by unanimous decision on June 8, 2012 at. Pierson was expected to face Dan Miller on September 22, 2012 at UFC 152. However, Miller pulled out of the bout after getting news about his son's kidney transplant operation and was replaced by Lance Benoist. Pierson defeated Benoist via unanimous decision. Pierson was expected to face Rick Story on March 16, 2013 at UFC 158. However, he was forced out of the bout with an injury and replaced by Quinn Mulhern. Pierson was expected to face TJ Waldburger on June 15, 2013 at UFC 161. However, Waldburger was pulled from the event and replaced by Kenny Robertson. Despite being hurt in the third round, Pierson won the fight by majority decision.
Personal life
Pierson lives in the town of Whitchurch-Stouffville, is married and has two sons. Pierson has at least temporarily left his job at the Toronto police force since they would not allow him to have a secondary source of income while being employed by the Toronto Police force. The police force was against Pierson's participation in these events because of the nickname 'Pimp Daddy' he had back in his early twenties. Sean Pierson has been involved with several other UFC fighters as part of a Toronto area anti-bullying program. Pierson is a top video game player having won tournaments and finishing third in a Canada Wonderland tournament.