Atlanta United advanced to the MLS Cup on Thursday, claiming a 3-1 aggregate victory over the New York Red Bulls despite falling to a 1-0 defeat against their Eastern Conference rivals.

Atlanta, who only made their Major League Soccer debut in 2017, will host the Portland Timbers in Major League Soccer's championship game at Mercedes Benz Stadium on December 8.

The Timbers advanced with a 3-2 victory at Sporting Kansas City, taking the Western Conference title 3-2 on aggregate after the teams played to a goalless draw in Portland on Sunday.

Diego Valeri scored twice, netting the game-winner nine minutes into stoppage time. Sebastian Blanco also scored for the Timbers, who have reached the MLS Cup for the second time in club history.

Atlanta had all but guaranteed their progress to the final after taking a decisive 3-0 lead in their first leg at home on Sunday.

"Once you move into the playoffs you can't take chances, you can't take risks, you have to defend as a team," Atlanta's veteran defender Jeff Larentowicz said afterwards. "I thought we did that tonight. It's a team effort in the playoffs."

Atlanta, coached by former Argentina manager Gerardo "Tata" Martino, never looked like relinquishing their advantage against the Red Bulls, who had finished the regular season on top of the East with the best record in MLS.

The aggregate defeat marked another frustrating campaign for the Red Bulls, who once again were unable to convert regular season dominance into playoff success.

Since finishing as runners-up in their only appearance in the MLS Cup in 2008, the Red Bulls have won the Supporters Shield -- awarded to the team with the best regular season record -- on three occasions, in 2013, 2015 and 2018.

But the Red Bulls are now left to reflect on another futile playoff campaign after a disappointing second leg display before their fans at the Red Bull Arena.

Tim Parker bundled in a consolation goal for the Red Bulls deep into stoppage time for the 1-0 win but it was too little too late.

With Atlanta lining up in a 5-4-1 formation, New York were unable to find a way through a resilient defence, with Brad Guzan in the visiting goal rarely threatened.

Instead it was Atlanta who had the best chances of the first half, with Venezuelan international striker Josef Martinez almost putting his team ahead in the opening seconds only to be denied by Red Bulls goalkeeper Luis Robles.

Robles was pressed into action again in the 21st minute, diving low to his right to beat away a shot from Julian Gressel.

With 10 minutes remaining, the Red Bulls looked to have grabbed a lifeline when Aaron Long bundled the ball out of Guzan's grasp and into the net.

But the goal was disallowed after a consultation from VAR.

Parker jabbed in his close range effort in stoppage time, but there was no way back for the Red Bulls.