The (Red Devils) have won just two of their last eight matches in all competitions and sit 11 points off fourth place in the Premier League.
Manchester United currently sit sixth in the Premier League table but have already lost five times in 17 matches, leaving them 19 points behind leaders Liverpool and 11 off fourth-placed Chelsea in the fight for Champions League places.
The 55-year-old, appointed in May 2016, won the League Cup and Europa League in his first season in charge, but leaves the club after a dismal start to the season culminating in the 3-1 defeat at Liverpool on Sunday.
“Manchester United announces that manager Jose Mourinho has left the club with immediate effect,” a statement read.
“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.”
Michael Carrick, who joined the club’s coaching staff last summer following his retirement, will assume the role of interim manager.
The 37-year-old former England international will be assisted by Kieran McKenna until the club can appoint an external caretaker manager, a process that is expected to be completed within 48 hours.