Erik Lamela's opener was cancelled out by Hornets forward Odion Ighalo in the first half at Vicarage Road, with the hosts then suffering a blow midway through the second period when Nathan Ake was dismissed for a foul on Lamela.
Despite their numerical disadvantage, Watford put us under pressure late in the game but then up popped Son with a clever finish at the near post from Kieran Trippier's cross to secure our third Premier League win on the spin.
Mauricio Pochettino made three changes to the side that beat Norwich just 48 hours beforehand. Two of those came at full-back with Danny Rose coming in for Ben Davies and Trippier making his first Premier League start for the club in place of Kyle Walker.
In midfield, Tom Carroll was preferred ahead of Christian Eriksen after notching his first Premier League goal when he came off the bench on Boxing Day.
Harry Kane led the line as normal having plundered 12 goals in his last 11 games for us, but we operated with a tweaked formation as Eric Dier slotted into a back three between Jan Vertonghen and Toby Alderweireld. Rose and Trippier played as wing-backs either side of Carroll and Mousa Dembele with Lamela and Dele Alli behind Kane.
There was a welcome return to the bench for Ryan Mason who has been short on luck with injuries this season so far.
Meanwhile, for the home side there were a couple of familiar faces to Spurs fans with Heurelho Gomes lining up between the sticks and Etienne Capoue taking his place in the centre of midfield.
Our change of formation seemed to outfox the Hornets as we enjoyed the majority of the early possession, although there were precious few chances inside the opening 15 minutes.
We soon grabbed the lead though Lamela. Watford defender Craig Cathcart slipped which allowed Lamela to run through and the Argentine international kept his cool to roll the ball past Gomes and into the bottom corner from the edge of the box.
The rhythm of the game continued in much the same manner as we made full use of the extra width in our system as Tripper and Rose hugged both touchlines and pushed forward at will.
Kane squandered a decent opening with 10 minutes of the half remaining as he opted to shoot with Lamela and Alli breaking alongside him.
It was the pocket in front of the Hornets’ back four that looked most profitable for us, with Lamela and Alli winning possession in dangerous areas.
Lloris had been a bystander for the entirety of the half but out of nothing the home side were level just before the interval. Ighalo pounced on a Troy Deeney header and wriggled away from Dier in the box before prodding the ball into the net.
It was a tough blow and one which was swiftly followed by another as Dembele appeared to hobble off, meaning Eriksen replaced him just a few seconds before the half-time whistle.
Kane did well to feed Lamela early in the second period and his dangerous cross was hacked away as we went in search of the lead again.
Lamela continued to find room on the break and Eriksen linked up well with Kane but the in-form striker wasn’t quite getting the rub of the green in the final third.
Kane was a whisker away from making it 2-1 just before the hour mark when he stooped to narrowly head wide under pressure from a lethal ball in from Trippier.
Shortly after came the main talking point of the half. Ake caught Lamela with a high tackle and after taking a minute to ponder his options, referee Anthony Taylor showed him a straight red card.
The move to 10 men prompted the home side to drop off, making it difficult for us to break them down.
As we patiently waited for an opening in the yellow banks of four, Eriksen almost found subsititute Son but Gomes did well to sprawl at his feet and nab it from him.
Watford put us under sustained pressure late on with a series of corners swung in by Ben Watson - and one almost paid off in the 89th minute when the ball appeared to be heading straight it at the near post, but Lloris threw himself across to block with his body.
Having weathered that storm, we got our reward in the 90th minute when Trippier crossed from the right after his initial ball in was repelled and Son flicked home at the near post with the inside of his boot for his first goal since scoring the winner against Crystal Palace in September.
The home side remained defiant, though, and it needed a heroic block from Dier in the six yard box to keep out a shot from Deeney in the 93rd minute.
The final whistle sounded soon afterwards and it was a job well done following a difficult outing.
Watford (4-4-2): Gomes, Anya, Cathcart, Britos (Prodl 60), Ake; Abdi (Behrami 45), Watson, Capoue, Jurado (Nyom 68); Ighalo, Deeney (c). Substitutes (not used): Arlakauskis, Guedioura, Holebas, Diamanti.
Spurs (3-4-2-1) Lloris (c), Alderweireld, Dier, Vertonghen; Trippier, Dembele (Eriksen 45), Carroll (Son 68), Rose; Lamela, Alli (Chadli 81); Kane. Substitutes (not used): Vorm, Davies Walker, Mason.
Goals: Watford – Ighalo 42; Spurs – Lamela 17, Son 90.
Yellow cards: Watford – Britos 50, Behrami 58, Deeney 62, Ighalo 65; Spurs - Alli 5, Alderweireld 80.
Red card: Watford – Ake 62.
Referee: Anthony Taylor.