The former French youth global, who was called up to the senior national team for the first time on 28 August, is expected back in the France squad’s training base in Clairefontaine immediately after finalising transfer formalities.
The young forward started his career at Lyon where he made his professional debut in December 2012, before joining Monaco in June 2013.
It completes a remarkable rise for Martial, who joined Monaco in 2013 for 5 million euros.
Louis van Gaal said he was “delighted” with his latest signing but warned that it could take the teenager time to adapt to the Premier League.
If Martial does not reach the levels suggested by his fee, Louis van Gaal’s boys would have to rely on Wayne Rooney up front, a player in the worst form of his career. I told him he is technical and powerful.
“We saw him at the Emirates last season playing with Monaco and causing trouble to the Arsenal defence, he has a bright future”.
The 19-year-old was initially preparing for France’s global friendly with Portugal before flying to Manchester to join the Reds, his third club in professional football after Olympique Lyonnais and AS Monaco.
However, fans hoping for another deadline day recruit were left disappointed as no further transfers were announced at Old Trafford.
James Wilson was linked with a loan move away from the club but that failed to materialise.
Chelsea complete the signing of Reading defender Michael Hector on a five-year contract.
Newport manager Terry Butcher said of the Wales Under-17 global: “In the past few days a number of clubs have shown an interest in Regan, but as soon as Manchester United came in there was only really going to be one outcome”.
He added: “The headlines make it sound like a huge, huge deal, but in the context of Manchester United it’s not wildly outside what they might normally expect to play for emerging talent”.