Mitchell has been working for the Blades for over eight years.
It was announced on Thursday he had left with immediate effect and by mutual consent.
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Stephen Bettis, United's chief executive, said: The timing of the change suits both parties and we are now working towards having a new team in place ahead of the summer transfer window.
On behalf of the board and the club, I'd like to place on record our thanks to Mitch for his hard work and dedication to the club. He is well-known and respected in the city and the football fraternity, and we wish him well."
Mitchell's contract was set to expire in the summer but he has departed early due to personal reasons after discussions with the United hierarchy.