Gillick was born to former minor league baseball player Larry Gillick in Chico, California. In 1951, he earned his Eagle Scout from the Boy Scouts of America. 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. After graduating from Notre Dame High School in Sherman Oaks, California, he hitchhiked to Vulcan, Alberta to toil as a kid pitcher with the semi-pro Vulcan Elks of the Foothills-Wheatbelt League. Gillick had to wire his grandmother for 25 dollars to finance his last leg from Montana to Vulcan. He attended USC and joined the Delta Chi Fraternity. He graduated in 1958 with a degree in business. 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 the Pittsburgh Pirates, venturing as high as Triple-A. A left-hander, Gillick posted a win/loss record of 45–32 with an earned run average of 3.42 in 164 minor league games.
Gillick retired from playing and began a front-office career in 1963, when he became the assistant farm director with the Houston Astros. 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. In 1976, he moved, this time to the expansion Toronto Blue Jays, becoming their Vice President of Player Personnel, and in 1977, their Vice President of Baseball Operations and General Manager. In 1984, he was named Executive Vice President of Baseball Operations.
As Toronto's general manager, Gillick won five division titles (1985, 1989, 1991, 1992 and 1993) and led the club to their first World Series championships in 1992 and 1993. 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.
In 1995, Gillick was named the general manager of the Baltimore Orioles to replace Roland Hemond, who had resigned. He cited the fact that they were close to winning a championship as a factor to his decision to come out of retirement. He guided the Orioles to the playoffs in 1996 and 1997. He resigned at the conclusion of his three-year contract in 1998. The Orioles struggled shortly after his departure, failing to achieve a winning season until 2012.
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. Upon his hiring, the responsibility fell on Gillick to trade Ken Griffey, Jr. to Cincinnati after Griffey played out his final season in Seattle. The Mariners made back-to-back playoff appearances for the only time in franchise history in 2000 and 2001, and the 2001 team, with a 116–46 record, tied the 1906 Chicago Cubs for the all-time Major League Baseball record for most wins in a single season. However, the Mariners failed to make it past the American League Championship Series in either year, and did not make the playoffs for the rest of Gillick's tenure as GM and advisor. The Mariners have not reached the playoffs since his departure.[unreliable source?]
Gillick was inducted into the Toronto Blue Jays "Level of Excellence" on August 8, 2002.
Gillick had permanent residence in Toronto with his wife Doris, however they have since re-located to Seattle after he became the Phillies GM. He had became a Canadian citizen in 2004.
Gillick retired from his position as general manager after leading the Phillies to a World Series championship in 2008. Assistant general manager Rubén Amaro, Jr. was named his successor. Gillick remained in the organization as a senior advisor to Amaro and Phillies president David Montgomery.[unreliable source?] In August 2014, Gillick became interim president of the Phillies while Montgomery was on medical leave. In January 2015, Montgomery returned but became Phillies chairman, while Gillick assumed the club presidency on a permanent basis.
^The list's only other MLB GMs were Boston's Theo Epstein (No. 3) and Oakland's Billy Beane (No. 10). Friedman, Dick (December 22, 2009). "2000s: Top 10 GMs/Executives". Sports Illustrated. Retrieved 2010-09-23.
^National Baseball Hall of Fame and Museum: Eras: Expansion, "Rules For Election For Managers, Umpires, Executives, And Players For Expansion Era Candidates To The National Baseball Hall of Fame  Retrieved June 24, 2013