 After his original loan spell, Mullins had already become a fans' favourite, but nine games into 2013–14, he had been given the nickname "Magnet Man" by the players and fans, a nod towards his ability in the opposition box.He was a member of the squad that achieved promotion to League One at the end of 2015–16, having also captained the club at Wembley Stadium in the 2016 Football League Trophy Final against Barnsley.On 23 May 2016, Oxford announced Mullins would be released upon the expiry of his contract, having made 149 appearances for the club, scoring nine times.