The former Pompey midfielder has signed a two-year deal at Field Mill.
Donohue was released from Fratton Park this summer after the Blues chose not to take the club option of an additional 12 months on his contract.
The 25-year-old joined Pompey from Chesterfield for an undisclosed fee in 2017.
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He made 47 appearances for Kenny Jackett’s side – but only 14 of them arrived last term.
Donohue was plagued with various injuries which meant he was unable to have a real impact in the failed League One promotion push.
However, the ex-Everton youth player will be hoping to make a swift return to the third tier at Mansfield after they suffered League Two play-off semi-final defeat at the hands of Newport.