The Old Trafford club matched the Premier League champion's bid and was prepared to offer improved terms, but the center back is bound for the Etihad Stadium.
Eliaquim Mangala has passed a medical ahead of a 48 million-euro move to Manchester City, despite late interest from rival United.
The Old Trafford club was willing to match the transfer fee and offer improved personal terms to the Porto center back.
But the 23-year-old underwent a medical at The Bridgewater Hospital in Moss Side on Saturday before departing Manchester on a private jet.
Mangala has now gone on a short vacation, but the Premier League champion is expected to announce the transfer in the next 24 hours.
City has pursued the center back for several months and completed the 15 million-euro signing of Porto teammate Fernando in June.
Mangala has three caps for France and was part of Didier Deschamps' World Cup squad, but did not feature in Brazil.
- By Paul Clennam
VAN GAAL TO ENHANCE MANCHESTER UNITED FACILITIES
New Manchester United manager Louis van Gaal is making a number of changes on and off the pitch ahead of the new season.
United has completed the signings of Ander Herrera and Luke Shaw from Athletic Bilbao and Southampton, respectively, and continues to pursue the likes of Angel di Maria and Arturo Vidal.
But the club is also busy preparing for a new regime under Van Gaal at its Carrington training base.
The Red Devils are in the process of installing floodlights at Carrington in order for the players to take part in afternoon training sessions later in the year.
United has also added new beds to the facility as Van Gaal makes plans for a sleep and rest zone for the players, who will have the option of heading home between training outings before returning for a 3pm session, or remaining at Carrington for the afternoon.
- By Paul Clennam