And just four minutes after the break the roles were reversed as Niang put it on a plate for captain fantastic to finish.
Watford ace Miguel Britos was unlucky to be sent off for a second bookable offence by referee Paul Tierney but it failed to tarnish the classy win.
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The on-loan star teased the Baggies defence before bending a shot past Ben FosterCredit: Reuters
Facts, stats, goals and lols...
M'Baye Niang must have listened to Watford manager Walter Mazzarri who last week told him he needs to do more to secure a permanent move from AC Milan. He netted after 12 minutes.
Troy Deeney was back after being on the bench following a virus against Sunderland on Saturday. He was on fire from the start, setting up Niang and then scoring himself.
The Hornets were also boosted by the return of the influential defender Sebastian Prodl from an ankle injury although he signalled to come off just before half-time clutching his ribs. It could cause more injury woes especially with Younes Kaboul the latest to be on the treatment table with a hamstring problem which could rule him out for the rest of the season.
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Deeney slid home after an assist from Niang that sunk West BromCredit: Reuters
Watford's victory over Sunderland on Saturday was their sixth league win at Vicarage Road this season, equalling their tally from 2015-16. This is the first time they have won more than six in a top-flight season since 1986-87, when they managed 12 under Graham Taylor.
The Hornets just keep chipping away, and keep popping up with a win when it looks like they might be sinking into trouble.
West Brom barely had a sniff of the ball in the first half with Chris Brunt forcing Heurelho Gomes into an easy save from a weak effort.
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The home fans celebrated with their side after they raced into an early leadCredit: Reuters
The Baggies have now won just one of their last 10 away games in the Premier League (D4, L4).
Top hitman Troy Deeney became the first Watford player to score in four consecutive Premier League home matches and took his tally to 106, fourth in the all-time list..
This was the first home league win for the Hornets against the Baggies since November 1994.
Miguel Britos was unlucky to be sent off for a second bookable offence by referee Paul Tierney.
Next five fixtures
Watford
8 April – Tottenham (A) – PL
15 April – Swansea (H) – PL
22 April – Hull (A) – PL
1 May – Liverpool (H) – PL
6 May – Leicester (A) – PL
West Brom
8 April – Southampton (H) – PL
16 April – Liverpool (H) – PL
29 April – Leicester (H) – PL
6 May – Burnley (A) – PL
13 May – Chelsea (H) - PL
What social said:
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Deeney raced on to Niang's ball over the top to drive the ball under Ben FosterCredit: News Group Newspapers Ltd
Deeney gave the home fans plenty to sing about after sinking West BromCredit: News Group Newspapers Ltd
What they said:
Watford manager Walter Mazzarri said: "I'm very happy for my team. The players played very well. It was a great game.
"I said before the game they were a good side and had two good results against Man United and Arsenal.
"It was a beautiful goal by Niang. Also the assist was good from Deeney."
West Brom manager Tony Pulis: "We gave two poor goals away. We still had chances and possessions. Chadli could come out another day with a hat-trick, so we are disappointed.
"The lads kept working but it wasn't our night. We needed a break and they had it."