Thomas Harley was rewarded for a solid first full season in the NHL when he signed a two-year, $8 million contract (average annual value of $4 million) with the Dallas Stars on Sept. 17, 2024. He led Stars defensemen in goals (15) and ranked second in assists (32) and points (47) in 79 games. He led the NHL at his position in overtime goals (three) and was tied for eighth in goals and led the Stars in plus-minus (plus-28) and ranked second in blocked shots (135).

The following season, Harley's 50 points (16 goals, 34 assists) led Dallas defensemen and his plus-minus (plus-32) was second to Esa Lindell's plus-34. His power-play goal 1:33 into overtime defeated the Winnipeg Jets 2-1 in Game 6 of the 2025 Western Conference Second Round and sent the Stars to the conference final for the third consecutive season.

Harley signed an eight-year, $84.7 million contract ($10.587 million) with the Stars on Oct. 28, 2025. He helped Team Canada win silver at the Olympic Winter Games Milano Cortina 2026.

He was one of three NHL defensemen in 2023-24 to score a power-play goal, short-handed goal and an overtime goal. He became the fourth-youngest defenseman in Dallas/Minnesota North Stars history to score an OT goal, which defeated the Seattle Kraken 4-3 at American Airlines Center on Dec. 18, 2023. He followed Julius Honka (21 years, 122 days April 4, 2017), Bob Rouse (21 years, 233 days Feb. 6, 1986) and Miro Heiskanen (22 years, 88 days Oct. 14, 2021). He scored overtime goals in consecutive games from Jan. 25-27, becoming the sixth defenseman in NHL history to do so.

The native of Syracuse, New York, was chosen by the Stars in the first round (No. 18) of the 2019 NHL Draft. He helped Canada win silver at the 2021 IIHF World Junior Championship, leading the tournament with a plus-14 rating.