Mullen competes in the S5 disability category in freestyle, backstroke and butterfly, specialising in sprint events.

[4] While at Temple he entered meets organized by Scottish Disability Sport, showing promise as a competitive swimmer.

[3] At the age of 10, after attending a gala in Dundee, he was approached by Paralympic champion swimmer Kenny Cairns, who believed Mullen had the potential to challenge at the highest level of the sport.

[3] In 2010 Mullen recorded times that saw him accepted into the British Swimming World Class Talent Programme.

The finals saw Mullen finishing fourth, just over half a second behind bronze medalist Zsolt Vereczkei of Hungary.

[4] Mullen was back in the British team the next year when he travelled to Montreal to compete in the 2013 IPC Swimming World Championships.