2008-09: Signed as an undrafted free agent with the Los Angeles Kings after completing four years of hockey with the University of Denver. Mullen improved on his point production each of his four years with the Pioneers, yet finished his senior year with only 25 points and a modest 81 points in 152 career games.
2009-10: played 44 games in the AHL with Manchester where he posted 4 goals, 6 assists and 16 PIM. He also suited up for one game in the ECHL.
2010-11: Mullen found himself in a utility role with Manchester in his first full AHL season – filling in at forward as well as providing a depth defenseman for the Monarchs. He skated in 67 games with the Monarchs and scored 3 goals with 17 assists; finishing with an even plus/minus and 32 PMs. Mullen skated in all seven playoff games in the Monarchs' first round series with eventual AHL champion Binghamton and was -5 with 1 assist and 4 PMs. Mullen was re-signed by the Kings to a one-year contract as an unrestricted free agent in July 2011.
Mullen is two-way defenseman who is neither overly physical nor particularly inclined offensively. He does have a good first pass and is capable of moving the puck up ice to forwards. He also is a fairly good lateral skater. Mullen however is easily overpowered and although he added some muscle over the past two seasons, is still too easily overpowered in the corners.
At this point in his career, Mullen in not much more than depth player for Manchester. He is a high energy skater that can play in a multitude of roles for the Monarchs.