Stephane Sessegnon scored on his West Brom debut to condemn former club Sunderland to their fourth Premier League defeat of the season.
The midfielder was gifted an open goal after Kieren Westwood initially saved well from Scott Sinclair.
Liam Ridgewell volleyed home midway through the second half before Morgan Amalfitano's angled effort made it 3-0.
Sunderland were forced to play the last 15 minutes with 10 men after losing Steven Fletcher to injury.
Some of the travelling fans showed their frustration at bottom club Sunderland's return of one point from five Premier League matches when manager Paolo Di Canio went to applaud them at the final whistle.
The Black Cats' only victory this term came with a 4-2 success over MK Dons in the Capital One Cup and, after a third-round meeting with Peterborough on Tuesday, they must entertain Liverpool and Manchester United in the top flight.
West Brom, in the relegation zone at the start of the day, had won only one of their past 13 games, while Sunderland had tasted victory only twice since January. Between them, the two teams had managed only four goals in the league this season.
Chances were at a premium at The Hawthorns, but not as a result of impressive defending - instead it was a lack of ideas and little confidence in attack that were the problem.
Nicolas Anelka, Sinclair and Amalfitano all looked lively for the home side without creating too many opportunities, while visiting hopes seemed to rest on Liverpool loanee Fabio Borini.
The striker was making his first start but flattered to deceive, some promising bursts failing to deliver an end product.
Ultimately, it was left for Sessegnon to open scoring against his former employers after receiving a work permit in time to play.
Despite Di Canio saying that Sessegnon "didn't care" for Sunderland, the Benin international offered no big celebration on scoring his first goal for new club Albion.
Adam Johnson should have equalised for the visitors but his driven effort was deflected inches wide, while at the other end Amalfitano, a threat on the right, saw his shot from the edge of the box ricochet over.
The visitors thought they should have had a penalty when an effort from substitute Jozy Altidore struck the hand of Jonas Olsson but referee Phil Dowd rightly awarded a corner.
Fletcher should have done better when he fired over with his first attempt and his disappointment was compounded by a shoulder injury caused by an awkward fall. As Di Canio had already made his three substitutions, the Black Cats were forced to play the rest of the game a man short.
West Brom capitalised moments later to wrap up their first win of the season.
Amalfitano crafted a cross for Ridgewell to volley home for his first goal since April 2012.
Frenchman Amalfitano, on loan from Marseille, then completed the scoring when his angled shot fizzed past Westwood and into the far corner.
West Brom manager Steve Clarke:
"When you have to wait this long for your first win there is some relief.
"It has been frustrating - particularly at home the performances haven't always been good. Today we got the breaks and we ran out deserved winners. We started slowly but I felt we played well after we got our noses in front.
"It's great that Stephane [Sessegnon] got off mark on his debut. He showed good respect to Sunderland fans by not over-celebrating. We brought him here to create chances and score goals so he's got off to a good start.
"Morgan [Amalfitano] was very good. I said he may surprise a few people. He's a good footballer. He creates chances and his goal was icing on cake."