Simon Nemec was selected by the Devils with the No. 2 pick in the 2022 NHL Draft. He was the first defenseman selected, and it was the highest a Slovakia-born defenseman ever was chosen.

Nemec made his debut with Nitra in Slovakia's top professional league as a 16-year-old in 2019-20, playing 12 games. He played the entire 2020-21 season for Nitra and had 19 points (two goals, 17 assists) in 37 games.

In 2021-22, Nemec had 26 points (one goal, 25 assists) in 39 games and was a standout for Slovakia during international tournaments. At the 2021 Hlinka Gretzky Cup, he was Slovakia's captain, had six points (one goal, five assist) and was named the tournament's most valuable player. He had six points (one goal, five assists) in eight games at the 2022 IIHF World Championship, the most for an 18-year-old defenseman in tournament history.

Nemec helped Slovakia win bronze at the 2022 Beijing Olympics, the country's first men's hockey Olympic medal. He had two assists in his NHL debut, a 6-3 Devils loss to the San Jose Sharks on Dec. 1, 2023, and completed the first hat trick of his NHL career by scoring in overtime for a 4-3 victory at the Chicago Blackhawks on Nov. 12, 2025. He became the eighth defenseman in NHL history to cap a regular-season hat trick with an OT goal and first since Jakob Chychrun for the Arizona Coyotes on April 4, 2021. At 21, Nemec was the youngest to do so, besting Chychrun (23 years, four days). He played for fourth-place Team Slovakia at the Olympic Winter Games Milano Cortina 2026.