Mourinho was told of his fate at around 9am on Tuesday in a face-to-face meeting with executive vice-chairman Ed Woodward at United’s Carrington training base.
The Portuguese, 55, took over in May 2016 and led United to League Cup and Europa League titles, but they are 19 points behind league leaders Liverpool.
The club have made a change after no progress with results or style despite spending nearly £400m on 11 players.
In the summer transfer window Mourinho routinely complained about a lack of signings. The Portuguese was unable to persuade the club’s hierarchy to sign a new centre-back, though they did sign £50m midfielder Fred, Porto youngster Diogo Dalot and Lee Grant as a backup goalkeeper.
Fractures in his relationship with Paul Pogba began to emerge in September and the France World Cup winner was stripped of the vice-captaincy before Sky Sports News filmed them having an animated disagreement on the training ground.
Mourinho has continued to drop hints that he remains unhappy. He now leaves after two-and-a-half years at the Old Trafford helm, with his final match in charge Sunday’s 3-1 defeat to arch-rivals Liverpool.
“Manchester United announces that manager Jose Mourinho has left the club with immediate effect,” a statement read.
“The club would like to thank Jose for his work during his time at Manchester United and to wish him success in the future.
“A new caretaker manager will be appointed until the end of the current season, while the club conducts a thorough recruitment process for a new, full-time manager.”
Mourinho leaves having won 84 of his 144 matches in charge.