[2] He was assigned to the Rookie-level Gulf Coast League Blue Jays for the season, appearing in 36 games and hitting .246 with 18 runs batted in (RBI).

[8][9] Prior to the start of the 2017 season, Jansen found he was having vision problems, and began wearing glasses on and off the field.

[10] The glasses paid immediate dividends, as Jansen hit .369 with five home runs and 18 RBI in 31 games for Dunedin before being promoted to the Double–A New Hampshire Fisher Cats.

He and Sean Reid-Foley became the first batterymates to debut in the same American League game since Billy Rohr and Russ Gibson did so in April 1967.

[15] Jansen hit his first major-league home run on August 14 off Royals pitcher Heath Fillmyer, breaking a 3–3 tie in a game the Blue Jays would end up winning 6–5.

Jansen spent time on the injured list in July and August with a right hamstring strain, but returned to the lineup to play the last 21 games of the season with a .322 batting average and seven home runs.

On June 6, Jansen was again placed on the IL after suffering a fracture in his left pinky finger after being hit by a 96-mile-per-hour (154 km/h) pitch during a game against the Kansas City Royals.

[20] On July 22, 2022, the Blue Jays set a franchise record for runs scored in a game with a 28–5 win over the Boston Red Sox at Fenway Park.

Jansen hit two home runs over the Green Monster in left field, scored four times, and contributed six RBIs.

[26][27] On July 27, 2024, Jansen was traded to the Boston Red Sox in exchange for minor-leaguers Cutter Coffey, Eddinson Paulino, and Gilberto Batista.

[39][40] In his youth, Jansen's family hosted players for the then-Seattle Mariners Single–A affiliate Wisconsin Timber Rattlers.