Heart of Midlothian Mullen joined the
Heart of Midlothian youth academy aged 12. He progressed to the under-19 squad and was promoted to the first team on 10 December 2011, when he was an unused substitute in the match against
Celtic. Mullen made his first team debut on 7 January 2012, in a
Scottish Cup tie against
Auchinleck Talbot; he took a
penalty kick that was saved by the Auchinleck goalkeeper in the first half. Hearts manager
Paulo Sérgio described Mullen as his man of the match. His next appearance came the following season on 26 January 2013, as he suffered a fractured foot during an
Edinburgh derby on 10 March, ruling him out of the
2013 Scottish League Cup Final. Mullen left the club at the end of the season after he was advised that he would not be offered a new contract.
Hibernian Mullen signed a one-year contract with
Hibernian, the
Edinburgh derby rivals of Hearts, in June 2013. He then went on to score a free kick in his first game, a friendly against
Gibraltar. Mullen failed to hold down a regular place in the Hibs first team, making just four appearances, with the last coming on 17 August and was allowed to leave the club in January 2014.
Raith Rovers On 10 January 2014, Mullen signed a contract with
Scottish Championship side
Raith Rovers until the end of the 2013–14 season. He made his debut the following day, playing from the start in a 0–0 draw against
Greenock Morton at
Cappielow.
East Fife Mullen signed a one-year contract with
East Fife in June 2014. He won League Two with them in 2015/16.
Cowdenbeath Mullen signed for
Cowdenbeath in 2016. He spent six seasons with the Blue Brazil, making over 200 appearances for them before leaving at the end of the
2021/22 season following their relegation to the
Lowland League .
Pollok Mullen signed for
Pollok in the
West of Scotland Football League in June 2022. He made his competitive debut in the first match of the
West of Scotland Premier Division season – a 3–1 win away to
Petershill. He was then named man of the match on his home debut the following week in a 0–0 draw with
Largs Thistle. A further man of the match award arrived in the Lok's 4–3 win at home to Annan Athletic in the Scottish Cup, where Mullen set up the first goal and scored a free-kick to make it 3–1.
International career Mullen has represented
Scotland at under-17 level. ==Career statistics==