The 21-year-old is going back to Tynecastle, where he scored once in nine games in the second half of last term before an unfortunate injury curtailed his progress with the Scottish Premiership side.
Mitchell, who joined United’s Academy at the age of 10, was named the Denzil Haroun Reserve-Team Player of the Year for his outstanding efforts during 2017/18.
The Mancunian played in 13 Premier League 2 games for United’s Under-23s last term, contributing four goals, before making his move north of the border in January.
Demi was named Man of the Match on his Hearts debut as they beat fierce rivals Hibernian 1-0 in the Scottish Cup.