 Mullen began to hold down a regular starting spot at right back for Adelaide including starting the inaugural FIFA Club World Cup game for the South Australian team in which he scored the equalising goal against Waitakere United.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.