Mitchell had reunited with Newcastle CEO Darren Eales, who recruited him at Tottenham Hotspur in 2014, but Eales will soon step down for health reasons.
“I’d like to thank everyone at Newcastle United for their support over the last year,” Mitchell said.
“I’m leaving at a time that is right for me and the club, particularly with Darren Eales – someone who I have worked so closely with in my career – moving on soon.
“The club is in great hands on and off the pitch, and is in a fantastic position to continue building.”
During Mitchell’s short tenure as sporting director Newcastle won the Carabao Cup – their first major domestic trophy in 70 years – before qualifying for next season’s Champions League on the final day of the Premier League season.
