The U.S. finished the game with only nine players after Brek Shea of FC Dallas received his second yellow card in the 60th minute and Chivas USA's Jorge Flores (playing in place of the suspended Anthony Wallace) was sent off in the 84th minute on a straight red card for a foul that occurred away from the ball.
First-half goals from Brek Shea and Tony Taylor gave the USA a 2-0 lead against El Salvador, which needed to beat the Americans to claim the other berth along with Honduras, which had beaten Jamaica, 4-0, in the first game of the doubleheader and finished second.
"Even though we only needed a tie tonight, we wanted to be aggressive," said U.S. coach Thomas Rongen. "We did a very good job in the first half of playing together and scoring the two goals. We played three tough games and put ourselves in a position to get to Egypt, which we did. We were able to determine our own fate in this game and I couldn't be happier with this team as they showed a lot of character through some adversity tonight."
On the first goal in the 30th minute, Club Brugge's Jared Jeffrey dummied Dilly Duka's driving cross to force El Salvador goalkeeper Diego Cuellar to overcommit and leave his net wide open for Shea to easily place the pass inside the far post.
With five minutes to go in the first half, FC Dallas rookie Peri Marosevic slipped a through ball behind El Salvador's back line and into the box, providing the speedy Tony Taylor, an early first-half sub for Sam Garza, time to take a touch and look up before slotting the ball inside the far post.
Brian Perk's shutout was his third in as many games for the USA, which qualified for the U-20 finals for a Concacaf record 12th time.
The shock of the Concacaf qualifiers took place in Group B, where Mexico has lost both games and is eliminated. Costa Rica and host Trinidad & Tobago are tied for first place with four points, one ahead of Canada, going into Wednesday's matches.
March 10 in Bacolet, Tobago
USA 2 El Salvador 0. Goals: Shea 30, Taylor 40.
USA -- Perk, Williams, Agbossoumonde, Davies, J.Flores, Ownby, Jeffrey, Duka (Maund, 82), Shea, Marosevic (Cruz, 69), Garza (Taylor, 21)
El Salvador -- Cuellar, Lopez, Menendez (Sosa, 77), Garcia, Alas, Chavarria, A.Flores, Orellana (Santos, 66), Blanco, Alfaro (Baires, 70), Alas.
Yellow cards: USA -- Shea 27, Shea 60, Flores 78; El Salvador -- Chavarria 25, Blanco 55.
Red cards: USA -- Shea 60, Flores 84.
Referee: Rene Guerrero (Nicaragua).