Susac attended Jesuit High School in Carmichael, California, where he played for the school's baseball team as a catcher. He signed a letter of intent with Oregon State University to play college baseball for the Oregon State Beavers baseball team, but was also coveted by Major League Baseball (MLB) organizations.Baseball America rated Susac as the 40th best available player in the upcoming 2009 MLB Draft. After Susac graduated from Jesuit, the Philadelphia Phillies selected Susac in the 16th round of the draft. Susac opted not to sign, instead enrolling at Oregon State.

Eligible for the 2011 MLB Draft, the San Francisco Giants selected Susac in the second round. Susac signed with the Giants, receiving a $1.1 million signing bonus. The Giants assigned Susac to the San Jose Giants of the Class A-Advanced California League in 2012 and the Richmond Flying Squirrels of the Class AA Eastern League in 2013. He hit .256 in Richmond with 12 home runs and 46 RBIs, earning an Eastern League All-Star selection, and was invited to the Arizona Fall League, where he adjusted his swing and improved to hit .360 with two home runs and 11 walks in 17 games. Susac was listed as MLB.com's #3 prospect in the Giants organization for 2014. Susac began the 2014 season with the Fresno Grizzlies of the Class AAA Pacific Coast League (PCL), where he batted .268 in 63 games through July 26.