He has signed a 2 year contract that has clauses in case his injury plays up.
Croft was training with Cardiff as he recovered from his series knee injury, the former Blackburn and Ipswich defender will take on a midfield role at Blundell Park.
He left Grimsby in 1996 for a record £1.7m to join Blackburn before a a £0.5m move to Ipswich.