Jankowski's grandfather, Lou Jankowski, was a 20-year professional who played 127 NHL games for the Detroit Red Wings and Chicago Blackhawks in the 1950s, and his great uncle is Hockey Hall of Famer Red Kelly.

What Jankowski didn't have until attending Stanstead College, a prep school in Quebec, was size; he was not selected in the 2010 Ontario Hockey League draft because he was 5-foot-8, but his play at Stanstead caught the notice of scouts and he grew to 6-foot-2.

The Calgary Flames went off the board and took Jankowski, rated as the 43rd-best prospect by Central Scouting, with the No. 21 pick in the 2012 NHL Draft. He's the highest NHL draft pick from a Canadian high school. Jankowski had already committed to play at Providence College. He steadily improved during his four seasons, ending his career with 40 points (15 goals, 25 assists) in 38 games in 2015-16. He was part of the Friars' NCAA championship team in 2015.

The Flames signed Jankowski to a two-year, entry-level contract on March 30, 2016. He spent almost all of the 2016-17 season with Stockton of the American Hockey League, finishing with 56 points (27 goals, 29 assists) in 64 games. He also made his NHL debut against the New York Islanders on Nov. 28, 2016.

Jankowski began 2017-18 with Stockton, but after scoring five goals in six games he was recalled by the Flames and scored his first NHL goal Nov. 9, 2017. He played for the Pittsburgh Penguins, Buffalo Sabres and Nashville Predators, the latter for three seasons until he was traded to the Carolina Hurricanes on March 7, 2025, for a fifth-round pick in the 2026 NHL Draft. Jankowski signed a two-year, $3.7 million contract (average annual value of $1.85 million) with the Hurricanes on May 11, 2026, two days after helping them reach the Eastern Conference Final.