Magee scored after 38 minutes to put the Galaxy in the lead, then added two more in the 68th and 74th minutes to seal his treble.
Robbie Keane then added a fourth with 10 minutes to play to complete the biggest win of the opening weekend.
Jamison Olave's own goal seven minutes from time gifted the Portland Timbers a point as they came from two goals down to draw 3-3 with the New York Red Bulls.
An Edgar Espindola double (9, 24), which sandwiched a reply from Diego Valeri (14), and Olave's (29) strike at the right end put the visiting Red Bulls in control at half-time.
But Darlington Nagbe pulled one back after 56 minutes and the unfortunate Olave inadvertently complete the comeback.
Alvaro Saborio netted in the 71st and 85th minutes to give Real Salt Lake a 2-0 win at the San Jose Earthquakes.