The 20-year-old, who made three starts for the club last season on loan from Meadow Lane, will travel with the Stags ahead of Saturday's clash at Wycombe.
Mitchell joins the club following the news that reserve goalkeeper Ian Deakin could be out for up to six weeks after dislocating a shoulder during the warm-up before last Saturday's home clash with Exeter.
Manager Paul Cox told the club's official website: "After Ian's unfortunate injury, we have had to make sure that we are adequately covered.
"Liam did well for us last season and has a good pedigree. He will give us both cover and competition for the goalkeeper's jersey."