Spence began new chapter in the NHL at age 24 when he was traded to the Ottawa Senators by the Los Angeles Kings on June 28, 2025, for a third-round pick (No. 67) in the 2025 NHL Draft and a sixth-round pick in the 2026 NHL Draft. He was chosen by the Kings in the fourth round (No. 95) pick in the 2019 NHL Draft and had 28 points (four goals, 24 assists) in 79 regular-season games and one goal in five Stanley Cup Playoff games in his final season with the Kings. Spence was fourth among Los Angeles defensemen in scoring and had one season left on the two-year, $3 million ($1.5 million average annual value) contract he signed with the Kings on Aug. 1, 2024.

Spence's 12 penalty minutes in 2024-25 were the lowest among any Kings skater (minimum 70 games) and he was one of five NHL defensemen to play at least 70 games and have 12 PIMs or fewer. He played his 100th NHL game April 15 to become the sixth defenseman selected in the 2019 draft to reach the milestone.

Before debuting with the Kings, Spence played 103 games for Ontario of the American Hockey League (AHL), where he was named to the AHL All-Rookie Team and AHL First All-Star Team. He was chosen to the Quebec Maritimes Junior Hockey League First All-Star Team in consecutive seasons from 2019-21 while playing for Moncton and Val-d'Or.

Spence is a dual citizen of Canada and Japan. He played for Canada at the 2019 Under-18 Men’s World Championship and the 2021 IIHF World Junior Championship, where he won a silver medal playing with future Kings teammate Quinton Byfield.