Mullen, who turns 21 on Tuesday, has agreed on a two-year contract extension, tying him to the Reds until the end of the 2012/13 season.

"It's a good feeling. I've had a pretty good year so far, the club has been really good to me, so I hope that I can keep developing here and improving," Mullen said.

"I've lived in Adelaide most of my life, it's a privilege to play for Adelaide United, and to be celebrating my 21st birthday with a new contract is pretty special as well."

Mullen, who made his A-League debut in 2008, will turn out in his 50th game for the club in Friday night's blockbuster against Melbourne Victory at Etihad Stadium.

"It's always a big game against Melbourne, and it will be another reason to try and get a win for the boys," Mullen said. "I can't wait to get out there on Friday night."

Mullen's signing gives has re-enforced Adelaide's defensive stocks with fellow defenders Iain Fyfe, Robert Cornthwaite and Nigel Boogaard all contracted to the club until at least the end of next season.