Prior to turning professional, Tkachuk played one season for the Boston University Terriers, earning All-Hockey East Rookie Team honors.

[2] In his play for the BU Terriers, he scored four goals and ten assists in 19 games as a freshman before making the U.S. under-20 national team for the 2018 World Junior Championships.

Tkachuk signed a three-year entry-level contract with the Senators on August 13, 2018, forgoing his remaining three years of NCAA eligibility.

[10] On October 17, after skating in four games for the Senators, it was announced that Tkachuk had a torn ligament in his leg and was set to be out for a month to recover.

[15] Tkachuk was selected to replace the injured Auston Matthews on the Atlantic Division roster for the 2020 NHL All-Star Game on January 22, 2020.

[21] On December 11, 2021, Tkachuk scored his first NHL hat trick against Brian Elliott in a 4–0 win over the Tampa Bay Lightning.

[23] At the outset of the 2022–23 season, Tkachuk was expected to play on the first line with Josh Norris and Drake Batherson.

Tkachuk scored a goal in the win, which helped move the Senators closer to a possible playoff spot in the Eastern Conference.

Tkachuk was selected to the U.S. under-20 national team for the 2018 World Junior Ice Hockey Championships in Buffalo, New York, winning bronze.

[32][33][34] His older brother, Matthew, is an alternate captain for the Florida Panthers, and his younger sister, Taryn, is an NCAA Division I field hockey player with the University of Virginia.