Early career Mullen played his first rugby league game aged seven for the Taree-Old Bar Lifesavers (now known as Taree Panthers). Two years later (after a brief stint playing soccer), Mullen relocated to Newcastle, where he continued his junior career with the Western Suburbs club. His first representative game was for St Therese's Primary School, New Lambton, in the Regional Knights Knockout competition, with the school winning the competition in 1996 and 1997. He played most of his junior football at five-eighth. Mullen joined the Knights as a junior in 1997. He played five-eighth for the NSW under-17s in 2004. While attending St. Francis Xavier's College in Hamilton, Mullen played for the
Australian Schoolboys team in 2004 and 2005.
2005 Coach
Michael Hagan picked Mullen for the first-grade squad aged 18. His first grade debut was on 15 May 2005 at
Energy Australia Stadium against the
Wests Tigers in round 10. Mullen played eight games for Newcastle in his debut year as the club finished last on the table.
2006 Mullen played 15 games for Newcastle in the
2006 NRL season as Newcastle qualified for the finals. Mullen played in the club's 50-6 elimination finals loss against the
Brisbane Broncos at the
Sydney Football Stadium.
2007 In 2007, Mullen was selected as starting five-eighth for the Knights. On 10 April 2007, the day after Mullen turned 20, Johns announced his shock retirement from rugby league due to a neck injury. Mullen became first-choice halfback. After playing nine first grade games as halfback and 31 games in total, he was a shock pick at that position for the
New South Wales Blues in the 2007
State of Origin series. Mullen was dropped after one game, a match in which NSW let slip an 18-6 halftime lead to lose 25–18. He would never manage to gain selection for New South Wales again.
2008-2016 Mullen continued to play for Newcastle and featured in the club's finals campaigns in 2009 and in 2013 where they reached the preliminary final against the
Sydney Roosters under the coaching of
Wayne Bennett. In 2015 and 2016, Mullen was part of the Newcastle sides which finished last on the table.
2017 During a pre-season training session in November 2016, Mullen was tested, with the A-sample testing positive for an
anabolic steroid and he was provisionally suspended on 17 January 2017 by the NRL. Mullen's B-sample tested positive to the steroid
Drostanolone, confirmed by the
Australian Sports Anti-Doping Authority (ASADA) on 7 March 2017 and he was handed a Notice of Alleged Anti-Doping Rule Violations. Mullen opted to challenge the matter with the anti-doping tribunal. On 2 May 2017, the NRL Anti-Doping Tribunal imposed a four-year ban on Mullen, with a period of ineligibility backdated to the beginning of his mandatory provisional suspension, which began on 17 January 2017. ==Representative career==