Long, 23 has made 123 appearances for Sheffield United since coming through the Academy system in 2011.
The Sheffield born shot stopper was a regular until the arrival of Simon Moore from Cardiff City at the beginning of last season, and joins the League One outfit on a season's long loan.
"He came through with a lot of expectations at Sheffield United, but last season did not go according to plan," Dons boss Neal Ardley said.
We would like to wish George all the best at the Kingsmeadow Stadium.