According to the Manchester Evening News, The 20-time English champions have long been following the 16-year-old. They claim the deal has been agreed between the two sides and the winger has already switched clubs.
The Old Trafford club have done business with Ajax in recent times, having signed Daley Blind for £13.8m ($17.5m). The Dutch club's sporting director Marc Overmars confirmed the transfer as completed by email.
In addition to this, Timothy Fosu-Mensah and Gyliano van Velzen were also signed from Ajax, with Tahith Chong being the latest acquisition from the Amsterdam club. Baars' Instagram profile says he is already a United player.
Before joining United, Chelsea and Arsenal were interested in signing the talented winger. His father even confirmed there were offers from Italy and Portugal, but the player's desire to play for the Red Devils saw him make a switch to United.
"We have had talks with Ajax that were between Millen and the club. But it was certainly nothing shocking, he was even appointed captain this season of the B1," Baars' father told Noordhollands Dagblad.
"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. 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," he said.
Baars will be eligible to play for the United U-18s in January, who are managed by Kieran McKenna. In recent times, the Premier League giants have added quality young talent to their squad and the list includes the likes of Chong, DJ Buffonge, Lee O'Connor, Nishan Burkart and Joshua Bohui.