Rookie Wells Thompson scored the winner in the 57th minute after veterans Pat Noonan and Taylor Twellman had tallied in the first half.
Arturo Alvarez and substitute Abe Thompson - with his first touch of the match - struck goals for Dallas, which rallied to tie the game, 1-1, when Alvarez scored, but trailed again at halftime and never caught up.
Noonan's poise, touch and sense of space troubled a FCD team plagued by defensive woes for much of the season. After scoring the opening goal when a deflected Steve Ralston cross reached him, Noonan set up Twellman for the second goal by nodding on a Ralston corner kick, and touched a low Twellman cross for Thompson to smash home the third.
Ralston played in the middle with Jeff Larentowicz and Andy Dorman to fill the absence of the suspended Shalrie Joseph. Thompson took Ralston's spot at right midfield.
FCD coach Steve Morrow had to revamp his personnel and formation as first midfielder Pablo Richetti, and then his replacement, Alex Yi, suffered injuries that knocked them out of action.
Each Revs' goal exploited defensive frailties: Adrian Serioux failed to block the cross Noonan put away; Noonan's header hit Carlos Ruiz but wasn't cleared; and Twellman chased down a long ball into the corner and centered a ball that Noonan redirected past three stranded defenders.
Morrow excised Brazilians Ricardinho and Denilson from the squad entirely yet still FCD conjured up a fair number of scoring opportunities against a Revs midfield missing the suspended Shalrie Joseph. Juan Toja covered a lot of ground, Dax McCarty moved smartly to plug lanes and support the attack, and Dominic Oduro and Alvarez raided in support of Ruiz.
Alvarez tied the game after 30 minutes with a blistering 16-yard strike set up by left back David Wagenfuhr's overlapping run, and nearly scored again a few minutes later via a one-two with Ruiz, but keeper Matt Reis charged out to block the shot.
That save grew in importance four minutes before halftime. Ralston crossed, Noonan's header hit Ruiz, and Twellman reacted first to bang in the loose ball from six yards out for his fourth Open Cup goal this season.
Richetti, apparently hobbled by a hamstring injury he'd been suffering from, left at halftime. Serioux moved to central midfield and Yi took his place in the back line. Early in the second half, Twellman ran onto a through ball and shot wide, but FCD didn't heed the alarm and surrendered another goal in the 57th minute.
New England broke upfield on a counter but a ball to Twellman forced him into the corner. FCD players got back but didn't mark up tightly nor fill spaces, and Noonan, his back to goal, simply touched Twellman's pass into the path of Thompson, who drilled it low into the net.
The other Thompson, Abe that is, replaced Yi six minutes later, and right after coming on put away a header from Clarence Goodson, who knocked the ball across the goalmouth after Serioux had served it from the right flank.
FCD stepped up the pressure, and Kenny Cooper made his first appearance since suffering a broken leg in early June. Oduro lashed a through ball into the net - not the first time the Revs three-man back line had been so breached -- but was adjudged, correctly, to be offside, and New England held out.
Losers at Pizza Hut Park in the 2005 and 2006 league championship games, the Revs finally broke through on what had been a field of horrors. Dallas, trapped in a four-game winless skid in league play, disappointed its fans once again.
U.S. OPEN CUP, final
Oct. 3 in Frisco, Texas
FC Dallas 3 New England 2.
Goals: Alvarez (Wagenfuhr) 30, A.Thompson (Goodson) 64; Noonan 21, Twellman (Noonan) 41, W.Thompson (Noonan) 58.
FC Dallas - Sala, Wagenfuhr (Cooper, 76), Serioux, Goodson, Moor, Ricchetti (Yi, 46; A.Thompson, 64), McCarty, Toja, Alvarez; Oduro, Ruiz.
New England - Reis, Riley, Parkhurst, Heaps, Larentowicz, Dorman, Smith, Ralston, Thompson (Cristman, 78), Noonan, Twellman.
Yellow cards: New England - Larentowicz 74, Riley 87.
Referee: Arkadiusz Prus