But he has been out on loan at Gainsborough and has played just twice for the Stags this season.
Somner, 28, moved to Field Mill from Notts County in August 2008 and made more than 60 appearances, but has been on loan with Altrincham this season.
Gary Mills, Rob Duffy and loanees Simon Grand, Peter Vincenti, Kevin Pilkington and Steve Cook have already left the club this month.
Source: BBC Sport
Source: BBC Sport