According to the Manchester Evening News, the 16-year-old has completed his move to Old Trafford and will be the new addition to their academy. He has long been linked with a move to the 20-time English champions.
Baars was part of Ajax's academy and his impressive display and the Dutch giants' youth system saw him attract interest from the Premier League clubs. He was on United's radar when his compatriot Louis van Gaal was in charge at Old Trafford.
The teenager was spotted first by United scout Marcel Bout when the latter was working under Van Gaal. The former Barcelona and Bayern Munich manager was shown the exit door in May 2016 and was replaced by Jose Mourinho.
Bout was retained after the arrival of the Portuguese international and continued his position at the club. Bout was the key figure in helping United land the talented youngster from Ajax and will follow Timothy Fosu-Mensah and Daley Blind from the Amsterdam outfit.
It was earlier reported that Baars had completed his switch to Old Trafford at the end of the last calendar year. However, the deal did not go through and it is now believed the forward has received an international clearance to seal the transfer.
United's Under-18s are in Austria for the pre-season preparation and Baars has already featured in the Red Devils Under-18s' 6-0 win over SV Ebensee. His father revealed last year that the player snubbed advances from Chelsea and Arsenal in order to complete the Old Trafford move.
"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," Baars' father explained.
"He played nine seasons in Amsterdam and he has always had the dream to play football in England. There was interest from Italy, Portugal and England, but when he was at Chelsea and United to look, he knew he wanted to play in Manchester. He did not even see Arsenal, his choice was made."