Diego Valeri opened the scoring just two minutes into the contest for his 100th goal in a Timbers shirt. Carlos Vela equalized in the 17th minute for the Black and Gold.
It looked like the two teams would share the points until Felipe Mora netted the match-winner in the 93rd minute to move Portland into sixth place, one spot behind their guests.
Story of the match
Valeri made his mark and hit his milestone just two minutes into the match as he chested down a long cross from Josecarlos van Rankin and finished past LAFC goalkeeper Tomas Romero from a tough angle.
He had a chance for a brace inside of six minutes when Marvin Loria sprinted up the right flank and flicked the ball with his left foot, putting the Argentine through in the box, but Romero gobbled it up after a heavy touch by Valeri.
Jose Cifuentes went for a highlight reel bicycle kick that missed and after a failed clearance attempt landed at the penalty spot behind his head, he tried the overhead kick but it went wide.
The Black and Gold would soon be level Diego Rossi ran down a long ball and served up a perfect cross to Vela, his left-footed effort from close range making it 1-1.
Timbers goalkeeper Steve Clark nearly conceded a sensational highlight in the 59th minute by coming way out of goal, giving Rossi a chance for a chip from some 35 yards out. Bill Tuiloma had to sprint to the goal line and clear it.
Three minutes later, Romero pulled off a sensational double save against Valeri as the Argentine unleashed one from inside the box, and after the ball was cleared only as far as Valeri, he trapped it and let it drop to his foot before firing, but Romero tracked it well, making a two-handed block with a full-stretch lunge.
In the 71st minute, the Black and Gold appeared to take the lead as a corner kick was headed on by Cifuentes and tapped home by Rossi right in front of goal, but the Uruguayan forward was whistled for offside.
The match seemed destined for a draw until the third minute of stoppage time when Mora got his head to a pinpoint cross from Sebastian Blanco and nodded it past Romero at the far post to give the Timbers all three points on a very special night.
Man of the match: Diego Valeri
His 100th goal was something for everyone at the club to cherish and he put in an energetic performance, helping his side to a much-needed victory.