As a player, Middlemass made 169 appearances in the Football League as a defender for Workington before a near fatal car crash ended his career.
After nearly a year out of the game, Middlemass returned to the side at Workington (then in the 3rd tier), but after a few appearances he realised his injuries had been too severe, and retired from professional playing.
In 1998, he joined Preston North End as chief scout to David Moyes, and remained for the tenures of Craig Brown and Billy Davies.
In his only role as a manager, Middlemass took over at Carlisle United in November 1987, converting a club that had suffered consecutive relegations from the Second to the Fourth Division, to one that narrowly failed to reach the promotion play-offs on goal-difference in 1990.
During this season, where Carlisle United had led the division for several weeks, he was complimented by the national press on the way he played an attractive style of football with three at the back.