Timbers finished two places and seven points behind Western Conference winners Dallas in the regular season but they made a lightning start in front of their own fans in only their second conference final, with Lucas Melano testing Jesse Gonzalez inside the opening 30 seconds.
With five minutes barely on the clock Melano gave Gonzalez another scare as the goalkeeper spilled his shot onto the post, and the Argentine livewire rolled an effort inches past the far post minutes later as Portland came out of the blocks well on the front foot.
Dallas, playing in their first conference final in five years, finally began to find a foothold in the game and had sights of goal through Kellyn Acosta and Mauro Diaz, but Adam Kwarasey was twice equal to the task in the Portland goal.
However, it was no surprise when it was the hosts who opened the scoring midway through the first half as Liam Ridgewell bundled home from point-blank range after Dairon Asprilla had flicked on a corner at the near post.
Nat Borchers was denied by Gonzalez as the home side went in immediate search of a second, and the Dallas stopper had to turn a header from Acosta over the crossbar to prevent an own goal just two minutes into the second half.
The pressure told eight minutes after the restart as Timbers doubled their lead in spectacular fashion, Asprilla catching Gonzalez off guard with a swerving effort from 25 yards as Portland established a commanding advantage in the two-legged tie.
However, minutes after Ryan Hollingshead had missed a glorious opportunity to halve the deficit for Dallas when he was put one-on-one, the visitors did find an important away goal when David Texeira got in front of his man at the near post to clip home.
That goal briefly gave Dallas the ascendancy, but there was a sting in the tail as Gonzalez could not hold a header from a corner and Borchers poked home in the first minute of injury time to extend Portland's home unbeaten run over Dallas to seven games and put them in control ahead of next Sunday's second leg.