Tkachuk followed both his father, Keith, and brother, Mathew, into a pro hockey career and enjoyed an excellent rookie season with the Senators in 2018-19. Chosen fourth overall by Ottawa in the 2018 NHL Entry Draft, the Boston University alum donned the number seven and cracked the lineup as an 18-year-old. Ranking among the league's rookie scoring leaders, he recorded 10 points over his first 10 outings and that stretch included a three-point performance against the Philadelphia Flyers in his second career game and the game-winning tally while facing the Dallas Stars. Later in the schedule, he dominated against the Edmonton Oilers – potting two goals which included the difference maker.