The Golden Knights will take on the Anaheim Ducks, who eliminated the Edmonton Oilers in six games on Thursday, in the Western Conference Second Round.
Brett Howden extended his goal streak to three games, and Carter Hart made 21 saves for the Golden Knights, who are the No. 1 seed from the Pacific Division.
Kailer Yamamoto scored, and Karel Vejmelka made 21 saves for the Mammoth, who were the first wild card from the Western Conference.
Howden opened the scoring at 15:02 of the first period. Marner's shot from the high slot went wide of the net, but the rebound caromed off the end boards right to Howden, who put it past Vejmelka at the left post to give the Golden Knights a 1-0 lead.
Marner made it 2-0 at 19:15 of the second period, scoring with a slap shot past the glove of Vejmelka from the top of the right circle.
Yamamoto cut it to 2-1 at 7:41 of the third period with a wrist shot from the right circle that snuck under the blocker arm of Hart and trickled into the net.
Colton Sissons responded back for Vegas at 9:40, picking up the rebound of Brayden McNabb's shot and scoring from the slot to make it 3-1.
Marner scored his second goal of the game on the power play to push the lead to 4-1 at 12:09. Shea Theodore's one-timer from the point was blocked before it reached Vejmelka, and Marner quickly located the rebound and scored from the slot.
Cole Smith scored an empty-net goal at 16:24 for the 5-1 final.
