Earnshaw's 65th minute leveller earned the Bluebirds a point after Jason Koumas' deflected shot had fired the Baggies into the lead after 49 minutes.
The City striker's 20th strike of the season came after Bernt Haas upended Mark Bonner in the box to send the Ninian Park crowd delirious.
But it was the performance which had manager Lennie Lawrence smiling at the end of 90 minutes.
"They're one of the favourites to win the league and rightly so," said Lawrence. "So from our point of view it gives us great heart because I thought we were a bit better than them for parts of the game.
"And even more so when you look at the players unavailable to us.
"And I think we could have won it if we had that little bit of luck at the end."
Richard Langley's free-kick seven minutes from time almost handed Cardiff their first win against a top-six side, but his curling effort smashed against Russell Hoult's woodwork.
But West Brom also had their chances to seal all three points following Koumas' opener. Cardiff keeper Neil Alexander had to be in top form to deny Koumas a second while his last gasp save at the feet of Danny Dichio kept the Bluebirds in the contest.
But Baggies boss Gary Megson was left disappointed after seeing his side throw away their one-goal lead.
"Before the game I would have been pleased to get a point because I see Cardiff as a top-six side and it's always a hard place to come," said Megson.
"But having gone one up I'm disappointed with the lads to have not won from that position."