The 43-year-old, who made 10 appearances for the club last season, will not turn out for the first-team next term, but plans to play for the reserves.
Millen, who is the oldest player to have played in the SPL, believes he can help the club's young players coming through their system.
"I won't be playing for the first team again," admitted Millen.
"I hope to contribute to the reserves and help the younger players develop.
"Playing with the younger boys is the most enjoyable part of my job because they listen, they show respect and work hard."