Merilainen was playing in his first game since Oct. 27 for Ottawa, which was coming off a 3-2 overtime win at the Philadelphia Flyers on Saturday.
Ridly Greig and Jordan Spence each had a goal and an assist, Dylan Cozens had two assists and Michael Amadio scored for the fourth straight game for the Senators (8-5-3), who are 3-0-2 in their past five games and 6-1-3 in their past 10.
Nick Schmaltz and Clayton Keller scored, and Vitek Vanecek made 21 saves for the Mammoth (9-7-0), who lost the final three games of a four-game road trip and are 1-5-0 in their past six games overall.
Greig gave the Senators a 1-0 lead at 13:59 of the first period. Fabian Zetterlund's initial one-timer from the right circle was blocked by Dmitri Simashev, but when Lawson Crouse tried to corral the rebound, it hit off the skate of Cozens and deflected up off Vanecek before landing in the crease, where Greig tapped it in.
Keller tied it 1-1 shortly after a power play expired at 17:33. He took a pass from Mikhail Sergachev in the neutral zone, skated down the left wing, and knocked his own rebound five-hole while driving to the net.
Spence put the Senators back in front 2-1 with his first goal of the season at 7:13 of the second period. He scored blocker side with a slap shot from the high slot following a turnover below the goal line by Dylan Guenther.
Nick Cousins extended the lead to 3-1 at 15:52, stuffing a wraparound under Vanecek’s left pad.
Schmaltz cut it to 3-2 at 18:56. He skated wide down the right wing, made a move to the net around Jake Sanderson, and slid a backhand under Merilainen’s outstretched right pad.
Jack McBain thought he had tied it 3-3 just 22 seconds into the third period, but the Senators challenged the play for goaltender interference, and the call was overturned after a video review.
Amadio then got inside position on McBain and deflected Thomas Chabot's point shot past Vanecek to make it 4-2 at 3:46.
Merilainen kept it a two-goal game at 17:28 by making a desperation paddle save on Kevin Stenlund while Utah had Vanecek pulled for the extra attacker.
