Son of Leeann and Steve Mullen, a former rugby league footballer who played for the Canterbury-Bankstown Bulldogs and Western Suburbs Magpies in the 1980s.

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.

Mullen played in the club's 50-6 elimination finals loss against the Brisbane Broncos at the Sydney Football Stadium.

On 10 April 2007, the day after Mullen turned 20, Johns announced his shock retirement from rugby league due to a neck injury.

[11] On 2 May 2017, [12] 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.

Mullen was selected in his first senior representative team, the Prime Minister's XIII, for a game against Papua New Guinea in 2006.

Outstanding performances on his return against competition leaders Gold Coast and Melbourne thrust him back into Origin contention, jostling for the halfback spot with Brisbane's Peter Wallace, Manly's Matt Orford and Parramatta's Brett Finch.

Agreed facts tendered to court stated Mullen purchased a total of 39 grams of cocaine from a co-offender between 23 and 30 November in 2018 for the purpose of supply and personal use.