The 23 year-old, and cousin of Newcastle Jets defender Daniel Mullen, trialled with the Sky Blues during the pre-season and scored in its 7-0 victory over Earlwood Wanderers.
Mullen, who has spent the past four years in the US College soccer system in North Carolina, said he is excited for his first year as a professional footballer at Sydney FC.
"It’s an absolute dream come true. I’ve worked hard over the last four years and now I’m fortunate enough to be involved in the biggest club, I think, in the A-League," Mullen said.
"I’m just excited and I want to give my best and help the team win the Championship this season. The opportunity to play in the Hyundai A-League is a great one, it’s growing in stature, and being an Australian boy I want to be a part of the Australian pathway."
"I’m looking to stamp my place in the team, I’m not here just to make up the numbers, I really want to push for my spot and earn my place in the team."
Sydney coach Graham Arnold said he was impressed by Mullen during his two week trial.
"It’s a great opportunity for a kid who’s a late developer and who’s a stand out in the South Australian State League," Arnold said.
"We’ve been monitoring him for a while and we had a few good tip offs about him. He came to train with us for two weeks and was very impressive."
"It’s great that the FFA have opened up the mature age rookie position and we’re glad to have him fill it. He adds depth to the squad, physically he’s in really good shape and he gives us some good competition for places. We’re looking forward to having him at Sydney FC."
The signing of Mullen as a mature age rookie is part of the salary cap changes implemented by Football Federation Australia for the upcoming A-League season.
