[4] After finishing his senior season with Harvard, Killorn was assigned to the Norfolk Admirals of the American Hockey League (AHL) at the end of the 2011–12 campaign.

On February 10, 2013, he recorded his first career NHL assist and point on a goal scored by captain Vincent Lecavalier, against the New York Rangers.

On February 16, Killorn scored his first goal on José Théodore in a 6–5 overtime win against the inner-state rival Florida Panthers.

[9] The Lightning would eventually get swept in four games by the sixth-seeded Canadiens and Killorn finished his first playoff year and series with a goal and an assist for two points in all four contests played.

[13][14] On March 23, 2017, Killorn recorded his 100th career assist on a goal scored by Jonathan Drouin in a 6–3 win against the Boston Bruins.

On November 3, 2017, the NHL Department of Player Safety fined Killorn $5,000 for an incident that occurred during a game against the New York Rangers the previous night.

[16] Killorn completed the 2017–18 campaign playing in all 82 games and recording 15 goals, 32 assists and 47 points as the Lightning as a team finished as the top seed in the East to return to the playoffs.

On February 19, 2019, Killorn recorded his 100th career NHL goal against Carter Hart in a 5-2 Lightning victory over the Philadelphia Flyers.

In the opening round of the 2019 playoffs, however, Killorn would be limited to one goal and no assists for one point in all four games as the Lightning would be on the receiving end of a four-game sweep by the Columbus Blue Jackets.

Killorn was nominated for the Bill Masterton Memorial Trophy by the Lightning following the season but did not win it or was named a top three finalist by the NHL.

[29] On October 28, 2021, Killorn recorded his 200th NHL assist on a goal scored by Andrej Sustr in a 5–1 win over the Arizona Coyotes.

[33][34] Killorn finished the 2022–23 season playing in all 82 games again and putting up career highs in goals (27), assists (37) and points (64).

[36] On December 2, Killorn recorded his 200th NHL goal on goaltender Ivan Prosvetov in a 4–3 win over the Colorado Avalanche.