Mullin joined Lokomotiv academy at 16 years, and two years later made it to the youth squad. In total, he played 53 games for Lokomotiv U21 and scored 23 goals.
He made his debut for Lokomotiv first team in the 2012–13 Russian Cup game vs Torpedo Armavir on 26 September 2012.
In March 2013, Mullin for the first time was included into team sheet for the League match vs Rubin. In the same month he was nominated for FC Lokomotiv Player of the Month and won this prize.