The winger, who played for Saint-Etienne in France until joining LAFC in August, netted the decider to give the team a second Supporters' Shield in four seasons.
Mexican striker and fromer Arsenal forward Carlos Vela, LAFC's captain, opened the scoring in the 51st minute but Portland Timbers had equalized in the 81st through Dairon Asprilla's goal.
LAFC notched their 21st win of the regular season and moved to 67 points, 12 ahead of second-place Austin FC for the Western Conference lead and three points ahead of Eastern Conference leader Philadelphia, with the season ending next Sunday.
With the victory, LAFC clinched the top seed, a first-round bye and home advantage in the upcoming playoffs as well as a berth in the 2023 CONCACAF Champions League.
First-year head coach Steve Cherundolo has guided LAFC to the top after a ninth-place showing in last year's Western Conference table and set a record for any first-year MLS coach with Sunday's victory.
It's also a step closer to another accolade for Gareth Bale, who joined the American side during the summer.
