Fitness Guru takes Birmingham post.
He is taking up the role of Head of Sports Science at the Championship club, after seven years with the U's.
Having been bought to north Essex by Geraint Williams, Carolan has worked under five other managers in north Essex and told the official website that he had enjoyed his time at the club.
"A lot has happened at the club in the past seven years on the football side and I've really enjoyed my time at Colchester United.
"I was approached by Birmingham City though and it was an opportunity that I couldn't turn down.
"I have met a lot of great characters in my time at the U's and I wish them all the best for the future."