McWilliam founded Upper Deck in 1989 and served as its chief executive officer until his death. The company is best known for its creativity with baseball cards, including those that had pieces of game-worn jerseys, bats swung during games, holographs and other features. More recently, however, the privately held company has struggled. In 2009, Major League Baseball awarded an exclusive contract for using its imagery to New York-based Topps, which is a direct competitor to Upper Deck. McWilliam also founded Jet Source, an aircraft fixed-base operation and aircraft charter and maintenance company located at Palomar Aiport. He practiced 11 years as a certified public accountant after earning his bachelor's degrees in accounting and economics from California State University at Fullerton. He was also known as an avid fisherman and was a key source of funding in the Adopt-A-Creek trout-stocking program in Bishop, Calif.