Mondaini, who was ineligible for four games, scored after receiving a Paulo Nagamura cross from the right wing and found the left post from inside the 6-yard box.
Mondaini was suspended after a May 7 tackle on Real Salt Lake's Javier Morales left the RSL midfielder with a broken ankle.
Chivas (4-4-5) used the goal to snap a three-game winless streak.
Portland, meanwhile, fell to 5-5-2 and remained without a road victory.
Costly malfunction causes beer flood at Boston-area brewery
Wisconsin business offering 'therapeutic cuddling' forced to close