He is the son of former National League president Warren Giles. His baseball career began in his father's former organization, the Cincinnati Reds, during the 1950s and he was among a group of Reds executives (including former Cincinnati general managerGabe Paul and MLB executive Tal Smith) who helped to found the Houston Astros when they debuted as the Colt .45s in 1962.The Sporting News' 1962 Official Baseball Guide and Record Book, published in the Colt .45s' maiden season, lists Giles as the club's traveling secretary and publicity director. Subsequently, he became promotions director, and focused on that role after the renamed team moved into the Astrodome in 1965.
Giles started with the Phillies as the vice president of business operations in 1969. He worked his way up in the organization, with stops as executive vice president and president, before becoming the chairman in 1997.
Giles was also part of the ownership group that bought the Phillies from the Carpenter family in 1981. The group is composed of Giles, David Montgomery, Claire S. Betz, Tri-Play Associates (Alexander K. Buck, J. Mahlon Buck Jr. and William C. Buck), and Double Play Inc. (John S. Middelton). Mahlon Buck passed away in 2011. The group reportedly purchased the team for $30 million. As of April 2008, the Phillies were valued at over $481 million by Forbes magazine.
Giles is also honorary president of the National League—the same job his father held on an official full-time basis. The honorary president's only task for the season is to present the Warren C. Giles Trophy—which is named after Giles' father, Warren—to the NLCS winner.