Lawson Crouse, John Marino, and Daniil But scored, and Clayton Keller had two assists for the Mammoth (21-20-3), who have won three of their last four games.
Ridly Greig scored and Leevi Merilainen made 18 saves for the Senators (20-17-5), who have lost two in a row and five of their last seven games.
Crouse opened the scoring at 3:59, firing a wrist shot past Merilainen off a pass from Keller to give the Mammoth a 1-0 lead.
Marino made it 2-0 at 7:20, collecting the rebound off a shot from Keller at the point and beating Merilainen with a quick slap shot on the glove side. It was the second goal Merilainen allowed on four shots.
Greig scored at 17:41, picking up the rebound of an Artem Zub shot from outside the right face-off dot. Greig spun around and jumped on the bouncing puck at the left side before beating Vejmelka with a wrist shot on the glove side to cut the lead to 2-1.
But scored at 5:41 of the third period, sneaking the puck under Merilainen in a scramble in front of the Ottawa net to give the Mammoth a 3-1 lead. The Senators challenged the play for goaltender interference but the call on the ice was upheld.
