Hammaker grew up living in many different locations due to his father's career in the United States Army, and attended Mount Vernon High School in Fairfax County, Virginia, where he played basketball, football, and baseball.

Hammaker received a full basketball scholarship to East Tennessee State University (ETSU) in Johnson City.

[1][2][3] In Game 7 of the 1987 NLCS, Hammaker, pitching for San Francisco, gave up a three-run homer in the second inning to José Oquendo, a utility infielder who had hit only one home run that season.

[4][5] While with the Giants, Hammaker and teammates Scott Garrelts, Dave Dravecky and Jeff Brantley became known as the "God Squad" because of their strong Christian faith.

[8] His youngest daughter, Annalee, is married to minor league player Josh Rolette, a catcher in the Cleveland Guardians' farm system.