Before Union: Underhill transferred to Union from St. Gregory's University in Oklahoma where he started for three seasons on the baseball team. He was named to the All-Conference team as both a sophomore when he hit .438 and as a junior when he hit .399 and stole 14-of-15 bases. Underhill holds seven records at St. Gregory's including hits per game, hits per season, runs in a game, runs in a season, on-base percentage, batting averaging in a season, and outfield assists. Underhill prepped at Little Axe High School in Norman where he was a four-year starter on both the baseball and basketball teams. In baseball, he was named All-State, team MVP, and MVP of several tournaments. Underhill earned All-Conference and All-Little City honors in basketball.