Tampa Bay Lightning (2012-2023)

On May 19, 2012, Killorn signed a two-year, entry-level contract with the Tampa Bay Lightning after completing his playing career with the Harvard Crimson men's ice hockey team.^[2]

During the 2012-13 lockout, Killorn played for the Syracuse Crunch of the American Hockey League then came up to the NHL for the Lightning after it ended. On February 10, 2013, he recorded his first career NHL assist on a goal scored by Vincent Lecavalier, against the New York Rangers. On February 16, 2013, Killorn scored his first goal against the Florida Panthers. Killorn finished his rookie year, the 48-game lockout-shortened 2012-13 season with 7 goals and 12 assists in 38 games.^[3]

On March 4, 2014, Killorn played his 100th NHL game in a 4–2 loss against the St. Louis Blues where he recorded a goal on Ryan Miller in the first period. Killorn finished the 2013-14 season, his second in the NHL, playing in all 82 games with 17 goals, 24 assists and 41 points recorded. Killorn made his playoff debut on April 16, 2014, in a 5–4 loss against the Montreal Canadiens, recording a goal on Carey Price and an assist on a goal scored by captain Steven Stamkos. On June 19, 2014, the Lightning announced that they had re-signed Killorn to a two-year contract extension.^[4]

Killorn with the Lightning in October 2013

Killorn played 71 games in the 2014-15 season, recording 15 goals, 23 assists and 38 points. During the 2015 Stanley Cup playoffs Killorn had a successful post season run with the Lightning, helping them reach the Stanley Cup Finals against the Chicago Blackhawks. The Lightning ended up falling to the Blackhawks in six games.^[5] Despite the loss, Killorn had a strong post season for the team, and also posted several records for former players from Harvard. Killorn's game 1 goal on Corey Crawford was the first goal scored by a Harvard alum in a Stanley Cup Finals game. Killorn added a second goal in game four on Crawford to become the first alum with multiple goals or assists in a Stanley Cup Finals series. His 2015 postseason scoring totaled (9 goals, 9 assists, 18 points in all 26 games played), which are all single-post season records among the twelve Harvard alumni who have played in the NHL postseason. Killorn's 10 goals over two post seasons is second only to Dominic Moore (11 goals) on the Harvard all-time list, which was achieved in only 30 playoff games to Moore's 88.^[6]

On October 12, 2015, Killorn recorded his 100th career NHL point with an assist on a goal by Valtteri Filppula in a 6-3 Lightning win over the Boston Bruins.^[7] On October 27, 2015, Killorn skated in his 200th career NHL game in a 0-2 Lightning loss to the St. Louis Blues.^[8]

On July 17, 2016, Killorn signed a seven-year, $31 million extension with the Lightning. In the 2015–16 season, Killorn appeared in 81 games with the team, recording 14 goals and 40 points. Killorn also skated in all 17 Stanley Cup Playoff games the prior year, scoring 5 goals, 8 assists and 13 points.^[9] On December 8, 2016, Killorn played his 300th NHL game in a 5–1 loss to the Vancouver Canucks. 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.^[10] Killorn finished the 2016–17 season with 81 games played and 19 goals, 17 assists and 36 points recorded.

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.^[11] On January 22, 2018, Killorn played his 400th NHL game in a 2–0 win to the Chicago Blackhawks. Killorn completed the 2017–18 season playing all 82 games and recording 15 goals, 32 assists and 47 points.

On February 19, 2019, Killorn recorded his 100th career NHL goal against Carter Hart in a 5-2 Lightning victory over the Philadelphia Flyers.^[12][13] On February 28, 2019, Killorn played his 500th NHL game in a 4–1 loss over the Boston Bruins. Killorn recorded his first career NHL hat-trick against the Washington Capitals on March 16, 2019.^[14] Killorn ended the 2018-19 season playing all 82 games and putting up 18 goals, 22 assists and 40 points.

Shortly before the 2019–20 season began, Killorn was named alternate captain for the Lightning after Ryan Callahan went into an early retirement due to a degenerative back disease caused by multiple back injuries in a short time span. On January 17, 2020, Killorn recorded his 300th point scoring a goal on Laurent Brossoit in a 7–1 victory over the Winnipeg Jets.^[15] The last 3 weeks of the 2019–20 season would eventually get canceled due to the COVID-19 pandemic and the playoffs would get postponed until the summer resulting in Killorn finishing the regular season playing in 68 games with 26 goals, 23 assists and 49 points recorded. The 26 goals was a career high and was on pace for scoring 30+ goals for the first time in his career at the time the NHL announced the last 12 games would be canceled. On September 10, 2020, Killorn was suspended 1 game by the NHL Department of Player Safety for boarding Brock Nelson during the Eastern Conference final playoff game against the New York Islanders on September 9, 2020, at Rogers Place.^[16] Killorn and the Lightning would eventually defeat the Dallas Stars in 6 games in the 2020 Stanley Cup Finals to clinch the Stanley Cup.

On February 20, 2021, Killorn skated in his 600th career NHL game in a 4–0 loss to the Carolina Hurricanes. Killorn became the sixth Lightning player in franchise history to skate in 600 games with the team.^[17] Killorn finished the 2020-21 pandemic shortened season playing in all 56 games with 15 goals, 18 assists and 33 points recorded. Killorn sustained a broken fibula in game 1 of the 2021 Stanley Cup Finals when blocking a shot from Montreal Canadiens defenseman Jeff Petry; he did not play the rest of the series although he tried to do so after returning to practice just days after the injury occurred.^[18] The Lightning went on to win the Cup for the second consecutive season.

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. On March 13, 2022, Killorn played his 700th NHL game in a 2–1 win over the Vancouver Canucks. On April 21, 2022, Killorn put up his 400th NHL point in an 8–1 win over the Toronto Maple Leafs. Killorn kept up his durability in the 2021-22 season by playing all 82 games again and putting up 25 goals, 34 assists and 59 points. His 34 assists were a new career high. Killorn and the Lightning would go to another Stanley Cup Finals and would go on to lose in 6 games to the Colorado Avalanche.

On April 1, 2023, Killorn played his 800th NHL game in a 5–0 win over the New York Islanders. 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).

Anaheim Ducks (2023-present)

On July 1, 2023, after 11 seasons with the Lightning, Killorn signed a four-year contract with the Anaheim Ducks.^[19]

After missing the first 10 games of the 2023-24 season due to a broken finger in the preseason, Killorn made his season and Ducks debut on November 5, 2023 in a 4-2 Ducks win over the Vegas Golden Knights.^[20] On December 2, 2023, Killorn recorded his 200th NHL goal in a 4-3 win over the Colorado Avalanche on goaltender Ivan Prosvetov.^[21]