The 18-year-old defenseman had one shot on goal and three takeaways in 20:37 of ice time.
"I think I played solid game," Nemec said. "Sometimes I made some mistakes, but it wasn't bad for first game. I think I was good in the defensive zone and I was a little bit surprised (by that), but it can be better."
Vitek Vanecek made 13 saves over the first two periods in his Devils debut after being acquired in a trade with the Washington Capitals on July 8. Vancecek signed a three-year, $10.2 million contract ($3.4 million average annual value) July 19.
Akira Schmid stopped all 11 shots he faced in the third period for New Jersey.
Nikita Soshnikov scored for the Islanders. Ilya Sorokin allowed three goals on 12 shots in 29:55, and Cory Schneider made 13 saves in relief.
Soshnikov gave New York a 1-0 lead at 8:04 of the first period, but Yegor Sharangovich (15:24) and Dougie Hamilton (16:42) responded with goals 1:18 apart later in the period to make it 2-1.
Jack Hughes gave the Devils a 3-1 lead 52 seconds into the second period, and Erik Haula scored at 17:48 for the 4-1 final.
