Pierre-Emerick Aubameyang put the Gunners ahead in bizarre fashion, as he closed down and deflected Ben Foster's clearance into the net. Within a minute, the home side's task was made all the more tougher, as Troy Deeney was given his marching orders for an elbow on Lucas Torreira.
Here's the remarkable moment the visitors went ahead (UK only):
Having been under pressure at times in the first period, Arsenal struggled to find their rhythm after the break too, as Watford looked to more likely of the two teams to score. Adam Masina came closest for the home side, as his long-range effort rattled the crossbar.
The win for the Gunners puts them one point behind third-place Tottenham Hotspur and ahead of Chelsea—who have played a game more—on goal difference. Manchester United are in sixth, two points behind Arsenal.
Leno on Track to Become One of EPL's Best Goalkeepers
Despite having a man advantage for much of the game, the Gunners still had to withstand some heavy pressure from the hosts. Goalkeeper Bernd Leno proved up to the challenge.
The German international made an excellent stop to keep out Craig Cathcart's fierce drive, flipping the effort onto the post, and then pounced to his left to keep out Etienne Capoue's free-kick.
Leno was also commanding under the high ball, as the Hornets adopted a direct approach as they sought to get back into the game.
While the Arsenal supporters would have liked to have seen more control from their outfield players in recent weeks away from home, their goalkeeper has been one of the standouts:
The LT Arsenal account summed up the significance of the stopper's first-half performance at Watford:
When Leno arrived at the club from Bayer Leverkusen, he did carry a reputation as an error-prone stopper. However, after some initial wobbles at the base of the side in his first few games, he's looked at home in English football's top flight.
James Benge of Football.London thinks Leno has been one of the smartest signings of the season:
While there are a number of high-class goalkeepers in the Premier League in the form of Ederson, Alisson Becker and David De Gea, Leno's consistency is putting him in close proximity to that elite bracket. If Arsenal can get a sturdier defence in front of the German, he's capable of reaching those levels next term.
Arsenal turn their attention to the UEFA Europa League on Thursday, as they travel to Napoli for the second leg of their quarter-final showdown having established a 2-0 lead at the Emirates.
Watford will be confident in their next Premier League outing, as they travel to already relegated Huddersfield Town on Saturday.