Haller, who arrived in a deal worth up to £45m from Eintracht Frankfurt in July, scored twice as Manuel Pellegrini’s side consigned the hosts to a third straight defeat this term.
The Hammers, who lost all three of their corresponding fixtures last term are 13th, albeit in a cluster of 10 clubs on four points that goes up to fourth.
The French forward said: “It’s really important to get a win. Now it’s done, we’re really happy with the result and the work from the boys.
“It was time to win a game to launch the season.”
Haller, who missed last weekend’s 1-1 draw at Brighton, opened his West Ham account with a tap-in from Felipe Anderson’s cut-back.
And he then acrobatically turned the ball home with an overhead kick from three yards after following up from a corner.
Mark Noble’s third-minute penalty set West Ham on their way to their first win of the season after Manuel Lanzini had been fouled by Abdoulaye Doucoure.
But Watford responded to level with a well-worked Andre Gray goal, with the forward drilling home a right-foot shot from Will Hughes’ pass.
However, the Hornets were punished for their wastefulness in front of goal with just three of their 23 shots finding the target.
Javi Gracia’s side, who sit bottom of the table, have now lost seven successive games in all competitions for the first time since 1999.
At the same stage last term Watford had collected nine points after an impressive start, but a pointless return this time around represents a complete reversal of fortune.
While they did at least register their first goal of the campaign, manager Gracia appears to have problems at both ends of the pitch to contend with.
Despite creating a multitude of opportunities, Gray’s 17th-minute strike was all they had to show for their endeavour.
Gerard Deulofeu’s influence as a creator was a plus, but he cannot be relied upon to be the prolific forward the club need to improve on last term’s 11th-place finish.
The former Barcelona man had seven shots on goal, with only a close-range strike that hit the West Ham crossbar finding the target.
