The try-scoring prop was at the bottom of a mountain of bodies to score the winning try just four minutes from time as Harlequins came roaring back from 23-15 down to win 25-23 at The Stoop on Saturday and book their place at Twickenham.
Northampton should have closed the game out when Lee Dickson scored to put the Saints eight points ahead with 14 minutes remaining.
But the Harlequins pack was dominant and Marler outstanding at the end of a week in which he was named in England's summer tour squad.
"Joe Marler has been a special talent for a long time," said Harlequins director of rugby Conor O'Shea.
"He is 21. He will still learn lessons but he learns quickly.
"In the loose, in his carries and in his tackling he is a very unique talent and England are lucky to have him.
"Our front row, well, what a performance. I wouldn't single Joe out when you have James Johnson on the other side of the scrum and Joe Gray in the middle."
It was not a classic semi-final by any means, with Evans and Ryan Lamb each kicking six penalties. The difference in the end was Lamb's missed conversion.
"Semi-final rugby is about winning," said O'Shea, who was confident Harlequins still had a chance to win when they fell 23-15 down.
"Last year we lost to them in the league 20-16 and Joe Marler was held up over the line.
"The difference between the inches of last year and this year in terms of belief is enormous.
"Last year, we didn't deserve to win. This year, we didn't play well but semi-finals are about winning and the belief those guys had to pull themselves back was just exceptional.
"I thought if we could get any sort of field position (we could come back from eight points down).
"Our scrum was so dominant. They took off their front row and the man strength was out of their team and we had a lot of man strength on the pitch.
"In the last 10 minutes we started to play the way we can."
For Northampton, it was a third consecutive Premiership semi-final defeat - and all the more frustrating for director of rugby Jim Mallinder given they had come so close.
"It was always going to be tricky but when you get eight points ahead, you think you have a chance," Mallinder said.
"If you watch some of Quins' recent games, you will be impressed by their scrum. It has improved enormously. They put us under a lot of pressure.
"We take enormous credit from being in three semi-finals in a row after where we came from (winning promotion from the Championship).
"We want to win big competitions and I am pretty confident we have the infrastructure from the board down and the players to do that."
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