The 23 year old, originally from Adelaide, trialled with the Sky Blues at the start of the 2015/16 preseason, scoring in a 7-0 win over Earlwood Wanderers at Leichhardt Oval and featuring in a 5-1 defeat of Macarthur Rams at Campbelltown Sports Stadium during August 2015. Mullen has spent the past four years plying his trade in the US College soccer system in North Carolina for Mars Hill University, and has Hyundai A-League ties through his cousin, Newcastle Jets defender Daniel Mullen, and national team ties through his uncle Joe Mullen a former Socceroo. Most recently Mullen represented Para Hills in the National Premier Leagues South Australia.