He previously served as the general manager of four MLB teams: the Toronto Blue Jays (1978–1994), Baltimore Orioles (1996–1998), Seattle Mariners (2000–2003), and Philadelphia Phillies (2006–2008).

He continued to stay involved in Scouting and received the Order of the Arrow's Vigil Honor mere months after winning the College World Series at USC.

[4] In 1956 while playing for Vulcan Elks, Gillick was picked up by George Wesley of the Granum White Sox to pitch in tournaments.

He was also a gifted pitcher, playing on the 1958 National Title baseball team at USC and spending five years in the minor league systems of the Baltimore Orioles and Pittsburgh Pirates, venturing as high as Triple-A.

He would eventually work his way up to the position of director of scouting before moving to the New York Yankees system in 1974, as a coordinator of player development.

Shortly after Gillick resigned in 1994, the Blue Jays went into decline, not finishing higher than third place until 2006, and failing to make the playoffs until 2015.

[9] Gillick then became the general manager of the Seattle Mariners, who had parlayed their incredible 1995 playoff run into a new ballpark and the financial resources to become a perennial contender.

On November 2, 2005, Gillick was named the Philadelphia Phillies' general manager, after which his first big move was to trade Jim Thome and cash to the Chicago White Sox for Aaron Rowand along with prospects Gio González and Daniel Haigwood, being a move which cleared the way for Phillies' Rookie of the Year Ryan Howard to become the permanent starter.

[14] Since 2016, Gillick served as part-owner of teams in the collegiate woodbat Great West League such as the Chico Heat and Yuba-Sutter Gold Sox.