Mike Banner also scored for the Fire (5-5-5), which won its second consecutive game after losing five straight in all competitions. Goalkeeper Sean Johnson made four saves for his first Major League Soccer victory in his first start.
Landon Donovan scored both goals for the Galaxy (12-3-4), which lost its first home league game and is 1-4-1 in its past six games. Donovan's first goal was the 100th of his MLS career.
Pappa gave Chicago a 1-0 lead in the fourth minute with a 13-yard shot inside the left post for his seventh goal.
John followed one minute later by firing a 6-yard shot between goalkeeper Donovan Ricketts and the right post.
Banner made it 3-0 in the 19th minute with his first of the season. Ricketts dove to deflect Pappa's 19-yard line drive with his left hand but Banner volleyed the rebound from seven yards.
Donovan converted penalty kicks in the 37th and 81st minutes for his fourth and fifth goals and tied Taylor Twellman for fifth place in MLS career goal-scoring with 101.
Johnson kept Los Angeles from scoring the tying goal in the 85th minute when he dove to his right to block Alan Gordon's 10-yard header with his right hand.
The Fire's Freddie Ljungberg, acquired from Seattle on July 30, made his team debut when he replaced Pappa in the 58th minute. Nery Castillo, whom the Fire signed Aug. 1, did not make the trip.