Adrian Kempe had a goal and an assist, and Anze Kopitar also scored for the Kings (14-8-7), who were coming off a 6-0 win against the Chicago Blackhawks on Saturday. Kevin Fiala had two assists, and Darcy Kuemper made 19 saves.
Clayton Keller had a goal and an assist, and Karel Vejmelka made 23 saves for the Mammoth (14-14-3), who have lost two in a row and six of their past eight.
Kempe gave the Kings a 1-0 lead at 7:50 of the first period. He took a pass from Fiala on a 3-on-2 rush, got the puck around Nate Schmidt just inside the zone, and drove down the slot before making a move and tucking a backhand past the left pad of Vejmelka.
Armia made it 2-0 at 10:08, scoring blocker side on a breakaway.
Brandon Tanev appeared to have cut the lead to 2-1 on his own breakaway at 11:08, but the Kings challenged the play for offside, and the call was reversed after a video review.
Dylan Guenther did make it 2-1 with a power-play goal 34 seconds into the second period. He beat Kuemper glove side with a one-timer from above the circles.
Kopitar pushed it to 3-1 at 3:27 of the third period. Kempe couldn’t control a backdoor pass with his skate, but the puck bounced in the slot right to Kopitar, who lifted a shot past Vejmelka’s glove from in tight.
Keller cut it to 3-2 at 12:15, roofing a backhand over Kuemper’s blocker from the right circle.
Armia scored an empty-net goal at 18:22 for the 4-2 final.