That was only the second effort on target all game.
And Townsend bettered that solo strike on 84 minutes by running and weaving from box to box and drilling home inside the near post.
Defeat dented West Brom's slim hopes of qualifying for the Europa League and left the Eagles three points and one place above the drop zone.
FACTS, STATS, GOALS AND LOLS
The Palace fans won the chant of the day after taunting former boss Tony Pulis, over his legal battle with the club. They shouted "We're taking your house...we're taking your house, Tony Pulis, we're taking your house."
Mamadou Sakho; brilliant one minute. Having a nightmare the next. The Eagles need stability but on the on-loan Liverpool defender doesn't give them any.
Christian Benteke took up the mannequin challenge only about three months after it was popular. The Palace striker needs to move about a bit if he's to give his teammates options.
Talking about old fashions, great to see Jonny Evans sporting a Terry Butcher-style bandage after taking a whack to the head. The Baggies defender was patched up and sent back out to battle.
Evans might have had a bloody head, but he was having a brilliant old-skool tussle with Benteke. The former Villa man was getting increasingly wound up.
Wilfried Zaha took his goal well and breathed life into this dull match. The Palace winger once again looked a class above his teammates.
Palace will need to check on Patrick van Aanholt's fitness. The defender left the pitch in the 62nd minute on a stretcher after injuring his foot with a mis-hit shot.
Best tackle of the match? Sakho's on referee Mike Jones was a belter in the 75th minute that floored the whistler, who was caught up in play.
Palace boss Sam Allardyce: "I said the new players would make a difference and they have done that.
"To put back-to-back wins together here against West Brom at the Hawthorns is a big statement.
"It gives the players the confidence to push on and get out of the position we're in. It was a quality performance and we deserved the three points.
"We're working in that bottom-seven league and trying to win that.
"We did the basics so well and the quality of both goals topped it. West Brom barely had a shot on goal."
West Brom manager Tony Pulis: "I'm not going to criticise the players because they have been fantastic this year. We've had an off day and I hope that is all it is.
"Why was it an off day? If I knew I'd be a genius. We've worked hard and trained hard this week. We were tepid right from the very beginning.
"It is human nature to switch off a little bit sometimes. People are people - sometimes they have good days, sometimes they have bad days."