Though United were shambolic, particularly after Anthony Martial's 28th-minute red card, the context of this victory made it all the more impressive and significant for Spurs.
In their fourth fixture in eight days, the visitors responded to going 1-0 down inside 90 seconds to Bruno Fernandes' penalty by shredding Ole Gunnar Solskjaer's wretched side.
Heung-min Son scored twice after missing the two midweek fixtures with a hamstring problem and so did Harry Kane, while Tanguy Ndombele and even Serge Aurier were also on the scoresheet.
Spurs were incisive and ruthless in possession – and combative and cynical out of it. For the first time, they looked like a classic Jose Mourinho side, albeit slightly too open defensively still. They should really have won by more.
It felt like a landmark win, one of those occasions – like Mauricio Pochettino's 5-3 victory over Chelsea on New Year's Day – where everything just seems to click into place, heralding an exciting new era.
Mourinho made eight changes from Thursday's 7-2 win over Maccabi Haifa, again underlining the strength of his squad, and it feels like there is so much more to come from Spurs.
There was no Gareth Bale, Giovani Lo Celso, Steven Bergwijn or new signing Carlos Vinicius, while Sergio Reguilon and Ndombele, both 23, are among the players who will only get better.
Mourinho proves relevance with big win
For Mourinho, it is hard to imagine a more satisfying afternoon. The Portuguese's two-and-a-half year spell at United was characterised by disharmony and turgid football and his dismissal in December 2018 was met with universal relief.
His acrimonious relationship with key players and underwhelming tactics added fuel to accusations that he was yesterday's man and no longer part of the vanguard of modern managers. With this result, Mourinho did not entirely prove his critics wrong, but he helped to continue to turn the tide.
The match also further highlighted the structural problems at his former club that Mourinho blamed for his struggles there. The win capped off a fine week for Mourinho, in which he has had to be at his pragmatic best to keep Spurs in the Carabao Cup and Europa League, despite an unforgiving schedule.
Given their run, it would have been easy to forgive a below-par showing here but instead Mourinho outclassed his successor comprehensively, to prove he can still make waves at the top of English football.
Full-backs underline Spurs' transformation
Nowhere is Tottenham's transformation more evident than at full-back, where Spurs have not looked so threatening since the days of peak Kyle Walker and Danny Rose.
The positions have been a problem area for the club ever since Walker was sold and Rose declined through injury and off-field travails. Mauricio Pochettino even used five different right-backs at the start of last season before his sacking.
Serge Aurier and Ben Davies were Mourinho's first-choices in the position last term but both had their limitations. In Sergio Reguilon, the £27.5million signing from Real Madrid, and Matt Doherty, Spurs have signed two new and impressive full-backs, while also improving Davies and Aurier.
The Welshman replaced the hugely impressive Son and earned a fourth assist in four days by winning the penalty for Spurs' sixth. On the opposite flank, Aurier followed his impressive display against Chelsea on Tuesday with another game to remember, finishing the best move of the afternoon for Tottenham's fifth.
Suddenly, Spurs look to have four impressive options at full-back.
Lamela highlights new nasty streak
A leading subplot of Mourinho's tenure is his mission to turn Spurs from a bunch of "nice boys" into "intelligent", nasty winners. To paraphrase the Portuguese on Amazon Prime's recent documentary: "Nice boys win nothing."
So there is little doubt Mourinho would have enjoyed Erik Lamela's part in Anthony Martial's dismissal after 28 minutes, which was arguably the most important moment of the match. The Argentine exchanged raised hands with Martial as United were defending a corner and went down clutching his face.
Lamela knew what he was doing and his behaviour was soft, cynical and even a little embarrassing. But, in the eyes of someone like Mourinho, it's part of the game and it worked.
In fairness, Spurs were hammering on the door before United were reduced to ten men and duly turned the screw with two more goals before half-time as United conceded four times in the first half at home for the first time in Premier League history.