After interviewing for positions with the Scott Paper Company and Quaker Oats and being encouraged by Rendell to apply for a job with the Philadelphia 76ers, Montgomery reached out to former Phillies ace Robin Roberts, whose son he coached at Germantown, about a job with the baseball club. Montgomery was hired as a member of the Phillies' sales department in 1971, becoming director of sales and marketing in the mid-1970s and the head of the business department by 1980. During the early 1970s, he was also the team's scoreboard operator.
Montgomery, along with co-owner Bill Giles, purchased the team in 1981 for $30,000,000 ($78,000,000 today); Giles, the primary owner, named Montgomery the executive vice president of the team. In 1997, when Giles left the team presidency to become the chairman and focus on winning a new stadium for the team, he recommended Montgomery to replace him as team president. In so doing, Montgomery became the first native of Philadelphia to run the club in over 60 years. One of his focuses with the team has been brand development, evidenced by the team's commitment to signing successful players to long-term contracts prior to the opening of Citizens Bank Park in 2004.
In August 2014, Montgomery took a leave of absence from the Phillies, while undergoing treatment for jaw bone cancer. When he returned to the team in January 2015, he assumed the title of chairman, while Giles became chairman emeritus. Pat Gillick, who served as interim president during Montgomery's leave, assumed the role permanently.