He has previously played for East Stirlingshire, Albion Rovers, Ross County, Hamilton Academical, Livingston and Ayr United, as well as junior clubs Pollok and Kilbirnie Ladeside.
Having been signed as a youth player with a number of clubs,[3] Mullin dropped down to the Scottish Juniors with Pollok and Kilbirnie Ladeside.
[4] His debut came as a second-half substitute for Gary Fisher, in a 2–2 draw with Airdrieonians in the Scottish Challenge Cup in July 2014,[5] whilst his first goal for the side was a 91st-minute strike in a 2–0 victory over Annan Athletic.
[7] Mullin signed a two-year deal for Scottish Championship club Livingston on 1 February 2016, for an undisclosed fee.
[21] On 31 January 2023, Mullin would depart his parent club Livingston by mutual consent, and would join Ayr United on a permanent deal the same day.