Robbie Keane led the way with three goals, and Omar Gonzalez celebrated his new contract in style as the Galaxy dismantled RSL.
The teams traded early second-half goals after a scoreless first. Joao Plata, set up by a nice Ned Grabavoy touch, showed a quick burst of pace and good close finishing to put RSL on the board, but Keane had it equal shortly after, displaying physicality and silky skill to finish from just outside the area.
Tha Galaxy nearly took the lead soon after, but Donovan's incisive effort was saved on the line by the chest of Chris Wingert.
They eventually did get a second, through Keane in the 66th. The Irishman propelled himself into substitute Sean Franklin's fast-break cross, and Nick Rimando was helpless in goal.
The Galaxy put daylight behind them with a goal from newly minted Designated Player Gonzalez in the 75th minute, as the towering defender outjumped Nat Borchers to head home Donovan's corner.
Keane rubbed salt in the would when he completed his hat trick a few minutes from time. Taking a ball over the top, the veteran chipped an onrushing Rimando at the edge of the penalty area.
RSL pulled one back late, with Robbie Findley bundling home following a free kick, but it was only a consolation on a night that was all about the Galaxy.
Despite the result, RSL remains on top of the MLS Supporters' Shield and Western Conference standings with 41 points. The Galaxy climbed into fourth in the West, with 37.