Hamilton put New Jersey ahead 1-0 at 8:58 of the first period, scoring on a one-timer from the top of the right face-off circle off a pass from Ondrej Palat.
Mercer made it 2-0 at 14:23, finishing a pass from Hischier past the extended left pad of Jarry on a 2-on-1.
Timo Meier pushed the lead to 3-0 at 2:54 of the second period when he tapped in a backdoor pass from Jack Hughes at the left post during a 5-on-3 power play.
Mercer then made it 4-0 at 19:26. Jarry couldn't cover the rebound of John Marino's point shot, allowing Mercer to swipe the loose puck into the open net after winning a battle with Kris Letang at the edge of the crease.
"He's just got, like, sneaky skill," Marino said of Mercer. "Little things that might go unnoticed, whether it's a board play or being strong in front of the net, like he was tonight. It might not look it sometimes, but he's got that sneaky skill. He's been great."
The Devils outshot the Penguins 18-6 in the second.
"I think in the second we had some chances to get ourselves back into it," Pittsburgh captain Sidney Crosby said. "I think we've done some pretty good things. I think there's games where we felt like we could've at least grabbed a point, if not two."
Mercer completed the hat trick to extend the lead to 5-0 at 2:52 of the third. After Jarry slid out of position while making a save on Michael McLeod, Hischier quickly got the rebound and sent a spinning pass to Mercer for a shot into a wide-open net.
"It was a great feeling," Mercer said. "I knew going into the third I had two there. The guys were saying, 'Go get another one. Get another one.'"