The son of longtime NHL power forward Keith Tkachuk didn't need much time to make his mark in the League after being taken by the Ottawa Senators with the No. 4 pick in the 2018 NHL Draft. He made the Senators out of training camp and scored two goals in his second NHL game, Oct. 10, 2018, surpassing his father and older brother, then-Calgary Flames forward Matthew Tkachuk, for the family's fastest first NHL goal.

Tkachuk scored 22 goals his first season, second among rookies to Elias Pettersson of the Vancouver Canucks, and had 15 goals in his first 48 games in 2019-20 to earn a spot on the Atlantic Division team for the 2020 NHL All-Star Game. On March 28, 2019, Tkachuk had 12 shots on goal to tie a Senators single-game record set by defenseman Jason York on Jan. 30, 1999, and his 20th goal tied Alexandre Daigle (1993-94) for Ottawa's single-season goal-scoring record by a teenager.. The success earned him a seven-year, $57.5 million contract with the Senators on Oct. 14, 2021, and named the 10th captain in their history Nov. 5.

He played for the runner-up United States at the 4 Nations Face-Off and won gold with Team USA for the Olympic Winter Games Milano Cortina 2026, the country's first in men's Olympic hockey since the 1980 Lake Placid Games. He played with Matthew at 4 Nations for the first time for the same team in the same game other than the 2023 NHL All-Star Game in a 6-1 win against Finland on Feb. 13. The Tkachuks were the 14th set of brothers to play on the same team in an NHL international tournament and fifth to do so with the United States, joining Derian and Kevin Hatcher (1996 World Cup of Hockey), Aaron and Neal Broten (1984 Canada Cup), Brian and Joe Mullen ('84 Canada Cup), and Curt and Harvey Bennett (1976 Canada Cup).

Playing his 300th NHL game, Tkachuk scored his 100th goal at 19:11 of the third period and No. 101 with 18 seconds remaining in overtime in a 3-2 win against the New York Rangers on Dec. 2, 2022.

Tkachuk scored 37 goals and had 294 hits in 2023-24, the most goals ever by a player who had at least 290 hits in a single NHL season.

Though Tkachuk was born in Scottsdale, Arizona, he grew up and played youth hockey in St. Louis after his father was traded to the Blues in 2001. He played two seasons for the USA Hockey National Team Development Program and one at Boston University, where he was selected to the Hockey East All-Rookie team and helped BU win the Hockey East championship.

Tkachuk was captain of the championship United States team at the 2017 IIHF Under-18 World Championship and helped the U.S. finish third at the 2018 IIHF World Junior Championship.