[2][3][4] His father is Chris Harrison, and his mother Kim played college field hockey for San Jose State University.

[5][6][7][8] His maternal grandfather is former left-handed major league pitcher Skip Guinn, who pitched in 68 games in parts of three seasons for the Atlanta Braves and Houston Astros in 1968–71.

[11] Harrison attended De La Salle High School in Concord, California in the San Francisco Bay Area.

[8] He was named East Bay Athletic League Pitcher of the Year, a 2018 MaxPreps National All-American, and Cal-Hi Sports All-State Underclass.

[24] He began the year with the High-A Eugene Emeralds, and in seven starts had 59 strikeouts in 29 innings (striking out half of the 118 batters he faced) and a 1.55 ERA.

[26][27] With the Class AA Richmond Flying Squirrels while almost five years younger than the average player in the league, as he was the youngest player in the history of the franchise and turned 21 in August, he had a 3.11 ERA in 84 innings (18 starts) with 127 strikeouts (6th in the league; as he struck out 36.4% of the batters he faced), for 13.6 strikeouts per 9 innings, and he generated a 41% whiff-and-miss percentage with his fastball.

[38] At 22 years old, having had his birthday ten days prior, Harrison was the youngest pitcher for the San Francisco Giants since Madison Bumgarner in 2009.

[39] His fastball reached 97.6 mph; the only other left-handed Giants starter who threw that fast a pitch in the pitch-tracking era (since 2008) was Carlos Rodón.