The victory put LAFC on 44 points and moved it ahead of Vancouver Whitecaps on goal differential for the seventh and final Western Conference play-off spot.
However, Vancouver has a game in hand and ninth-placed Real Salt Lake (42 points) has played two fewer games than Los Angeles, so plenty of work remains for Bob Bradley's side.
Still, LAFC will be pleased with the all-around effort as Jamal Blackman turned in a clean sheet and three different players found the net.
Brian Rodriguez opened the scoring in the 21st minute, slipping a shot past Stefan Frei, while Latif Blessing added a critical goal in first-half stoppage time and Cristian Arango sealed the win from long distance in the 51st minute.
Seattle remains atop the Western Conference on 58 points but continues to struggle as the play-offs approach, with three losses and a draw from its last four games.