The North Londoners were 3-2 winners in Mourinho’s first game in charge , with Alli playing a starring role in the victory.
Spurs raced into a 3-0 lead at the London Stadium, with Son Heung-min firing past hapless home goalkeeper Roberto for the opener.
The South Korean then teed up Lucas Moura for Tottenham’s second just before the break, with Harry Kane adding a third when he headed home a Serge Aurier cross in the second half.
With pressure mounting on home boss Manuel Pellegrini, West Ham were at least able to mount a fightback in the final quarter of the game.
Michail Antonio pulled one back for the Hammers with his fourth goal against Tottenham in four different venues, and after Declan Rice correctly had a goal ruled out by VAR, defender Angelo Ogbonna scored another in stoppage time.
It was all too little too late for the hosts though, as Mourinho guided Spurs to a first Premier League away win in 306 days.
And after the game, he heaped praise on Alli, who has been out of form in recent weeks.
“I spent a few minutes with him in training, and also in our social life outside the pitch, and we were speaking that the best Dele has to be back,” he told BT Sport.
“He is too good to not be in the national team. He is too good to not be a fundamental player for Spurs.
“He is too good to not be one of the best players in the world, so he has to start now too, without injuries, hopefully, have some stability, and he is a fantastic player.”