During his two seasons in Stanstead, Jankowski quickly gained height, growing to 6'2" tall, while it was his play that caught the attention of scouts.[2] He was eventually selected in the 2012 OHL draft, taken as the 11th pick in the 7th round by the Saginaw Spirit though he declined to join the team.[6] The National Hockey League (NHL)'s Central Scouting Bureau ranked Jankowski as the 43rd best prospect for the 2012 NHL Entry Draft in its final ranking, an improvement from his 74th place rank in previous reports.[7] Other analysts ranked him higher, including Craig Button of The Sports Network, who placed him 41st.[8] The Calgary Flames selected him in the first round, 21st overall. He is the highest-ever draft pick out of a Canadian high school.[9] The Flames received some criticism for going "off the board" and selecting Jankowski well above his expected draft place, but assistant general manager John Weisbrod stated that while Jankowski's talent was "raw", other teams were certain to select him in the first round if the Flames did not. General manager Jay Feaster argued that Jankowski was one of the most talented players available in the draft.[7] At draft time his stats were 6'2.5", 168 pounds.
Jankowski initially committed to play for the Dubuque Fighting Saints of the United States Hockey League (USHL) in 2012–13 before for Providence.[4] However, he decided to bypass the junior league and move directly to college following the Flames' 2012 rookie development camp, stating he believed he was ready despite the fact that he will be one of the youngest players in the National Collegiate Athletic Association (NCAA).[10]
Professional
Jankowski began his professional career on March 30, 2016, signing a two-year entry-level contract with the Flames. The deal began during the 2016–17 NHL season; for the remainder of 2015–16, Jankowski played with the Flames' American Hockey League affiliate, the Stockton Heat.[11] These games did not count towards his contract, which was slated to expire following the completion of the 2017–18 NHL season. Jankowski was called up by the Flames in November 2016, making his NHL debut against the New York Islanders on November 28, 2016.[12] On November 9, 2017, Jankowski scored his first career NHL goal against the Detroit Red Wings in a 6–3 win.[13] On April 7, 2018, the last game of the Flames regular season, Jankowski recorded his first four-goal game in a 7–1 win over the Vegas Golden Knights.[14]
After a largely disappointing 2019–20 season, in which Jankowski was limited to a fourth-line role and contributed with just 5 goals and 7 points in 56 games, he was released by the Flames as a free agent after he was not tendered a qualifying offer to retain his rights on October 8, 2020.[15]
With the following day being the opening of free agency, Jankowski was signed to a one-year, $700,000 contract with the Pittsburgh Penguins.[16]
On September 14, 2021, Jankowski was signed to a professional tryout contract (PTO) by the New Jersey Devils.[17] After attending training camp and participating in their pre-season, Jankowski was released by the Devils without a contract offer. On October 15, 2021, he agreed to begin the 2021–22 season with the Rochester Americans of the AHL, signing a one-year deal with the primary affiliate of the Buffalo Sabres.[18] After collecting 5 goals and 12 points through his first 13 games with the Americans, Jankowski was signed to a one-year, $750,000 contract, with the Buffalo Sabres on November 28, 2021.[19]
As a free agent from the Sabres, Jankowski was signed to a one-year, two-way contract with the Nashville Predators on July 14, 2022.[20] On March 8, 2024, he signed a two-year contract extension with the Predators.[21]