 Early in his playing career, Millen won the Scottish Challenge Cup twice with Hamilton.During his spell with Kilmarnock he won a Scotland B cap against Wales B.

Millen earned fame later in his career for continuing to play at a senior level despite his advanced professional age.When he played for St Mirren against Dundee United on 5 May 2007 at Tannadice, he became the oldest player to play in the Scottish Premier League.