Will Hughes slotted home the opener for the visitors with 19 minutes on the clock, sweeping home Marvin Zeegelaar’s centre.
In the first half’s dying seconds it was two, when another Zeegelaar cross was inadvertently deflected into the net by DeAndre Yedlin.
And Andre Gray’s 62nd-minute strike made sure of the win for Watford, who condemned Rafa Benítez’s Magpies to a fourth straight Premier League defeat.
Benítez confirmed on Friday that Mikel Merino would return to his squad after recovering from a back problem, but opted to start Mohamed Diamé in the middle of the park alongside Jonjo Shelvey, with Isaac Hayden suspended.
United, with Jacob Murphy making his first start at St. James’ Park, began brightly. The former Norwich winger’s cutback found Joselu, but the Spaniard couldn’t connect properly with his shot.
Another charge forward from Murphy brought about another opportunity, with Matt Ritchie picking up the summer signing’s pass on the right. He cut inside, but saw his low shot clip the sidenetting.
The pace of Watford’s frontmen Gray and Richarlison was always going to pose a problem for the Magpies’ backline, and when Richarlison did get a sight of goal after being released by Abdoulaye Doucoure, Rob Elliot got down quickly to tip the Brazilian’s effort past the post.
But on the visitor’s next foray forward, they took the lead. Christian Kabasele fired a pass over to Zeegelaar on the left, and his low cross was swept home by Hughes.
Marco Silva’s side continued to push after the goal with Gray forcing Elliot into a save, but Newcastle, while struggling to open up Watford’s defence, were working hard to find a leveller. Joselu was played in by Ritchie, but he was denied by the fingertips of Heurelho Gomes.
But in time added on at the end of the first half, Watford extended their advantage. They played their way into a good position on the left flank, and the impressive Zeegelaar’s cross’ nicked off Yedlin and wrong-footed Elliot, creeping in it his near post.
Barely two minutes after the restart, Gray squandered a chance to put the game beyond Newcastle. Zeegelaar sent a pass down the left and the former Burnley man scampered clean through on goal, but skewed his shot wide of the target with only Elliot to beat.
And minutes later, a mistake from Florian Lejeune let in Richarlison, but he took too long to apply the finish and, after something of a scramble, United were able to clear.
They didn’t survive for too long, though, as just after the hour Silva’s men added a third. Richarlison was the architect, keeping the ball in play before sending a teasing cross into the box which was dispatched by Gray, sliding in at the far post.
Joselu forced a save from Gomes before being withdrawn, but Merino’s return – as a second half substitute alongside Aleksandar Mitrović, who saw a shot blocked late on – was one positive to take on a forgettable afternoon for the hosts, who now head to West Bromwich Albion on Tuesday night.
