Jason Kreis's men, who had already qualified for the playoffs before this evening, now lead Portland Timbers by two points, although Caleb Porter's side will clinch the supporter's shield if they beat Chicago Fire in their final fixture on Sunday.
Timothy Melia was also on top form to deny Alvaro Saborio in the 28th minute, but RSL had their breakthrough in the 49th minute when Edgar Mejia handled inside the area, and Alvaro Saborio sent the keeper the wrong way.
Their lead was doubled just three minutes later thanks to a composed side-footed finish from Joao Plata from just outside the box.
Julio Morales grabbed one back for Chivas, but it was not enough to stop them falling to a fourth consecutive defeat.
They take on the Timbers in their final outing of the season, although they have already been cut adrift at the bottom of the league and are consigned to a last-placed finish.