Career
During his long career, Millen played primarily as a defender for Street Johnstone, Alloa Athletic, Hamilton Academical, Kilmarnock, Hibernian, Raith Rovers, Ayr United, Greenock Morton and Clyde. He continued to play for Street Mirren in the Scottish Premier League aged 42. Millen played for Queen"s Park in the Scottish Third Division, aged 45.
When he played for Street Mirren against Dundee United on 5 May 2007 at Tannadice, he became the oldest player to play in the Scottish Premier League.
He broke the record set by Aberdeen goalkeeper Jim Leighton, who was aged 41 years and 302 days when he last played in the SPL. Gus MacPherson argued that Millen"s record is more notable because Millen played in the more physical role of an outfield player. Millen took control of the Street Mirren team for a match against Motherwell in November 2007, after a family bereavement prevented Gus MacPherson from attending the match, which ended in a 1–1 draw.
Millen announced his intention to retire from first team football in June 2008. Millen"s last league appearance was against Hearts on 15 March 2008, which meant that he set the SPL record for oldest player at 42 years and 279 days, almost a whole year more than the previous record.
Millen continued working as assistant manager to Gus MacPherson after retiring as a player.
After Danny Lennon was appointed as manager, however, Millen left the club to allow Lennon to appoint his own backroom staff Millen was appointed as Billy Reid"s new assistant at Hamilton Academical in July 2010. He left the club in December due to a contract dispute.
Due to an injury to James Brough, Millen re-registered as a player to sign for amateurs Queen"s Park in January 2011.
Millen was appointed assistant manager of Queen of the South on his 46th birthday on 10 June 2011. Millen parted company with Queens on 30 April 2012, alongside MacPherson, after the club was relegated to the Scottish Second Division.
He then became assistant to Mark Roberts at Ayr United and was appointed caretaker manager in December 2014, after Roberts was sacked. Millen was appointed Development Squad Manager at Greenock Morton in May 2015.