The 25 year old, who can also play in central midfield, is due at Crewe today to become their second signing of the summer.
The Alex have already snapped up Mitchel-King's former Histon team mate Patrick Ada on a free transfer. Mitchel-King is also on a free after 163 games in three-and-a half years for the Blue Square Premier League Club.
Thordarson said: "He is someone I have kept my eye on since late last season when he was recommended to me. "He came up here last week and I had a good chat with him. Now hopefully, this afternoon, he will be our player."