Wambach, who scored in the 122nd minute of Sunday's penalty kick victory over Brazil in the quarterfinals, raced in from the top of the penalty area worked inside Laure Lepailleur to head in Lauren Cheney's corner in the 79th minute to give the United States a 2-1 lead.
Alex Morgan added another three minutes later to add the insurance and extend the Americans' unbeaten record against France to 13 with a 12th victory.
"It's a rainy day, the ball is slippery," Wambach said. "It's all about who wants it more. Cheney scores a big goal. Alex Morgan seals the deal for us."
The United States next will face Japan, which beat Sweden 3-1 in other semifinal Wednesday, in Sunday's title match. The Americans have have never lost to Japan in 25 matches, winning 22.
Until Wambach's goal, France had the better of possession and more numerous chances.
The United States went ahead in the ninth minute. Carli Lloyd used a backheel flick to spring Heather O'Reilly free down the left side. O'Reilly outraced Laura George and drove a low cross toward the goal box, where Lauren Cheney was able to slightly deflect the ball at the near post and into the far side netting.
But the Americans didn't get another shot on goal until the 70th minute when Megan Rapinoe, off a free kick, drove a low ball that France keeper Berangere Sapowicz blocked but had to scramble to cover the rebound.
The French outshot the United States in the first half 13-8, but almost all of its tries were from distance. It continued to dominate early in the second half and equalized in the 55th, capitalizing on a prolonged period of possession and chances.
Sonia Bompastor settled a cross-field pass from Sandrine Soubeyrand, and curled a ball into the penalty area. With Gaetane Thiney making a diagonal run in front of U.S. keeper Hope Solo, the ball skipped once off the misty rain-slicked turf and into the goal off the right post.
The Americans started to gain some measure of control midway through the half with the switch of Cheney to the center of midfield and the replacement of Lloyd with Rapinoe, who pressed on the left side, causing turnovers and forcing the French to defend for a change.
Both teams played 120 minutes in their quarterfinals, but France had an extra day's rest and was able to control possession, holding 55 percent of it according FIFA statistics.
But Wambach's goal appeared to swing the momentum, enabling Morgan to get behind the defense and chip Sapowicz for the clinching score.
Despite being outplayed for much of the match, U.S. coach Pia Sundhage attributed the victory to an intangible attitude.
"A big heart,' she said. "I think France played very well. In the beginning, we did well, but we lost our legs. We had heavy legs out there. We had to make adjustments in the midfield. We did some coaching on the bench. I'm exhausted."
The Americans will play in the final for the first time since outlasting China on penalty kicks in the Rose Bowl in Pasadena, California.