Spurs completely bossed the game at Old Trafford and should have been more than 1-0 in front when United captain Bruno Fernandes was sent off for a foul on James Maddison in the 42nd minute.
Brennan Johnson had fired Spurs ahead in the third minute following a stunning assist by Micky van de Ven.
Dejan Kulusevski and Dominic Solanke then added goals against United's 10 men in the second half but Spurs also missed multiple big chances to score more.
Sunday's result left United with just seven points from their first six games in the Premier League this season.
Jamie Redknapp has said that Manchester United have hit 'rock bottom' after another defeat
In his post-game analysis for Sky Sports, former Spurs and Liverpool captain Redknapp tore into Erik ten Hag's United side.
'It gets to a point where you feel almost sorry for the manager, he looks out of his depth,' Redknapp said.
'It was so easy for Tottenham. It feels to me now, £616 million spent, United are going backwards. They are rock bottom right now.
'It's embarrassing. A club of this stature shouldn't be producing performances like that. They don't want it enough.'