The 16-year-old reportedly agreed to the switch in October and now the player's father, Stefan, has confirmed the news.
Baars, who is a winger and captain of Ajax's Under-17 side, was invited for tours around Chelsea, United and Arsenal, but called off a visit to the north London side after being convinced to move to Old Trafford.
"We have had talks with Ajax that were between Millen and the club," Stefan Baars told Noordhollands Dagblad. "But it was certainly nothing shocking, he was even appointed captain this season of the B1.
"Ajax is a great club with a very good youth, but they can unfortunately not cope so very well. You saw in the past with other youth players who now play at other clubs. But Millen was ready for a different challenge.
"He played nine seasons in Amsterdam and he has always had the dream to play football in England at Manchester United.
"He did not even see Arsenal, his choice was made."