Career
Livingston
Mullen signed for Livingston at youth level in 2012 after being released by Aberdeen. After progressing through the youth ranks at Livingston, Mullen made his debut for the first-team on 18 August 2012 against Airdrie United as a second-half substitute at the age of 17. In total, he made 24 appearances in all competitions in his first season at the club, with most coming as a substitute.
