[1] Heiskanen played hockey until the age of 14 in the youth program of the Finnish Liiga club Espoo Blues.

[4] HIFK head coach Antti Törmänen described him as his team's best defenceman that season, praising his ability to always make the right decisions.

[8] Heiskanen was named to the Stars' roster to start the 2018–19 NHL season[9] and made his debut on 4 October in a 3–0 win over the Arizona Coyotes.

[10] On 25 October, in a 5–2 Stars win over the Anaheim Ducks, Heiskanen scored his first career NHL goal, beating goaltender John Gibson off a pass from Jason Spezza.

In so doing, at 19 years and 99 days, Heiskanen became the second-youngest Stars defenceman to score a goal in the franchise's history.