The Giants selected Hjelle in the second round of the 2018 Major League Baseball draft.

[2][3] Hjelle is tied with former pitcher Jon Rauch as the tallest players in MLB history.

[11] Hjelle began the 2019 season with the Augusta GreenJackets of the Single–A South Atlantic League, and was their Opening Day starter.

[13] In August, the Giants promoted Hjelle to the Richmond Flying Squirrels of the Double–A Eastern League,[14] with whom he finished the year.

[18] On September 28, Hjelle earned his first career win after pitching four innings of one–run ball against the Colorado Rockies in a 6–3 victory.