Jankowski, who turned 24 in June, will be in uniform on Wednesday as the Padres conclude a three-game series with the Braves at Petco Park.
Jankowski, who was the 44th overall pick in the 2012 Draft out of Stony Brook University, was having a monster season between two Minor League affiliates.
He was hitting .335/.413/.425 with 32 steals this season between Double-A San Antonio and Triple-A El Paso.
"He put himself on our road map and made it easier to make this call," Preller said. "He earned his way up, for sure."
In parts of four seasons, Jankowski is hitting .