He played in Adelaide's Champions league semi final's 1st leg 3–0 victory over Uzbekistan outfit Bunyodkor and was surely the best game in his short career.

Mullen scored his first league goal for Adelaide with a powerful header in the 3–2 victory over North Queensland Fury in Townsville in Round 4.

He had some trouble holding a spot the first team squad with Robert Cornthwaite and Iain Fyfe regularly used at centre back.

However, since there were no spaces remaining in the club's A-League squad, Mullen was only eligible to participate in Western Sydney Wanderers' 2014 AFC Champions League campaign during the 2013–14 season.

[9] On 18 March 2014, following a long-term injury to Golgol Mebrahtu, Mullen was added to Western Sydney Wanderers' A-League squad.