As a sophomore for the Stony Brook Seawolves baseball team in 2011, Jankowski earned first-team All-America East Conference honors after hitting.355, setting a school record for stolen bases and going the entire season without an error. That summer, Jankowski was invited to play for the Bourne Braves, where he led the Cape Cod Baseball League in hits, runs, and triples, earning league most valuable player honors. In 2012, Jankowski had a break out year, breaking his previous school record for stolen bases and setting school records in batting average, hits, runs, and triples.
Jankowski was drafted in the first round of the 2012 Major League Baseball Draft by the San Diego Padres.
He was signed by the Padres" Northeast Scouting Director, Jim Bretz. After just two games with the AZL Padres, he was promoted to the Fort Wayne TinCaps, the Padres" Class A affiliate, for the remainder of the 2012 season.
With the TinCaps, Jankowski hit.282 with 17 stolen bases in 59. He spent the 2013 season with the Class A-Advanced Lake Elsinore Storm.
On August 21, 2015, Janikowski made his MLB debut.
In his first major league at-bat, he recorded a single against the Saint Louis Cardinals" John Lackey. On August 21st, 2015, Jankowski had his 2nd at-bat go for an Reserve Bank of India single. He would then later on, produce a run when Yangervis Solarte would hit a 2 run blast to Right Field.
Jankowski is the first ever Padre to record 2 hits in his first 2 at bats in Padres history.