A deal worth an initial £14.1million (€16m) is understood to have been agreed.
The 28-year-old Dutchman joined Manchester United from Ajax in the summer of 2014.
But, since Mourinho's arrival, Blind has seen his playing time substantially decrease with the Portuguese coach seeming unsure as to his best position.
United kept clean sheets in all of his starts, but with just one year left to run on his contract, a sale had been mooted for several months. He was part of four title winning teams with Ajax.
GETTYDaley Blind Manchester United confirm defender's return to Ajax on four-year deal
Blind leaves United with FA Cup, League Cup and Europa League winners' medals in his collection, while Old Trafford chiefs will be pleased to be United a profit on the player they signed for a £13.8m four years ago.
United flew to Los Angeles on Sunday evening and Blind was not with the travelling party as the Red Devils expect him to complete his move away from the club in the next week.
Blind made 141 appearances for United across all competitions, seeing time at center-back, left-back and midfield.
"I think it's no secret that we [Ajax] are still looking for a central defender and, if possible, it would be a great addition for Ajax to have a player with the quality of Daley to fit in the team".
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Shaqiri has indicated that he wants the move which should be sealed before the Reds head off to the United States as part of their pre-season plans.