McDonnell moves to the Stags on a season-long loan.
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Forest teammates Joe Gardner and Kyle McAdam have already moved to Mansfield this summer.
"There seems like a real family environment here with a strong bond amongst the players and staff. I feel like this is a club where I can really contribute and develop," McDonnell said.
"I've previously spoken to Kyle and Joe and they've said nothing but great things about the club."