Turnbull has left the club by mutual consent.
He arrived at Valley Parade in December 2020, shortly after the Bantams appointed a new chief executive, Ryan Sparks, who quickly sacked manager Stuart McCall.
Turnbull then oversaw a huge overhaul of the squad which brought an uplift of results under caretaker managers Mark Trueman and Conor Sellars, but having been given the job "permanently" in February, the pair were sacked in May.
That prompted another complete revamp of the playing staff under new manager Derek Adams.
After two hectic transfer windows, Sparks says the League Two club has decided to move in a different direction.
“I would like to thank Lee for his effort and support over the past 10 months," he said.
“A decision to move in a different direction with regards to our recruitment has been made, and I would like to wish Lee the very best for the future.
“Lee is a friend of our football club and will always be welcome back at Valley Parade. We have parted on good terms, and I know he is right behind the club moving forward.”
Adams will announce a restructure to the club’s recruitment department in the coming weeks.