The Red Devils announced the decision to part ways with the 55-year-old on their official website and via Twitter:
A statement read:
"Manchester United announces that manager Jose Mourinho has left the club with immediate effect.
"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—who was given a two-year contract extension in January—leaves United sixth in the Premier League table after 17 games. They are 19 points behind league leaders Liverpool and have picked up just seven wins.
The decision means a new man will be in charge for the remainder of the campaign, starting with the trip to Cardiff City in the Premier League on Saturday. The Red Devils are looking to keep in touch with the top four in the table and progress in the UEFA Champions League—they will play French giants Paris Saint-Germain in the round-of-16 after the draw on Monday.
Mourinho started life at Old Trafford in reasonably strong fashion, winning the UEFA Europa League and EFL Cup in his first season.
Despite finishing sixth in the Premier League that year—a place lower than they achieved in Louis van Gaal's final season—they had improved on their performance by earning three more points.
Last season, United gained a further 12 points to finish second on 81, albeit they were still 19 behind champions Manchester City with a goal difference of 40. They also reached the FA Cup final but lost to Chelsea.
Despite the improving results, the team's style of play—particularly their defensive approach to big games—did little to impress supporters. They haven't kicked on this season, and the team has resembled a rabble at times, enduring their worst start to a Premier League season.
Defensively, they've been shambolic:
Despite this decision, Paul Hayward of the Daily Telegraph believes there's a lot of work still to be done at United:
Per BBC Sport's Alex Bysouth, since leaving Porto in 2004, Mourinho's third seasons at clubs have typically been characterised by a lower level of performance from the team and the manager's relationship with both his players and the media becoming increasingly fractious.
History has therefore repeated itself for the Portuguese. United have not only struggled on the pitch, but after spending the summer complaining at the club's transfer activity, or lack thereof, Mourinho has also had numerous clashes with his own players and the press.
Now all eyes will be on United to see where they turn next in what is now a crucial point in the club's recent history.