He served as an assistant district attorney in Montgomery County from 1970 to 1977 and as an assistant public defender in Bucks County. In 1971, Greenleaf was elected as a Commissioner for his hometown of Upper Moreland Township, Pennsylvania. After one term, Greenleaf was elected to a seat in the Pennsylvania House of Representatives in 1976. He served a single term in the house before winning his bid for the State Senate in 1978. He was reelected seven times. Greenleaf considered a run for U.S. Congress in 1993, briefly forming an exploratory committee to challenge Marjorie Margolies. However, he dropped out before the county endorsement convention. In 2000, Greenleaf ran for Congress against incumbent Joe Hoeffel. Greenleaf had represented much of the eastern portion of the congressional district for almost a quarter-century. Ultimately, Hoeffel won the race with nearly 53% of the vote to Greenleaf's 46%. Greenleaf did not have to give up his state senate seat to run for Congress; Pennsylvania state senators serve staggered four-year terms, and Greenleaf was not up for reelection until 2002. In his last term, Greenleaf was Chairman of the Senate Judiciary Committee and serves on the Appropriations, Banking & Insurance, Consumer Protection & Professional Licensure and Environmental Resources & Energy Committees. Greenleaf chose not to run for re-election in 2018. He endorsed his son, former Montgomery County controller Stewart Greenleaf Jr., as his successor. However, Stewart Jr. was defeated by Democratic challenger Maria Collett. Greenleaf continues to serve as a partner in his law firm, Elliott Greenleaf, whose attorneys include Montgomery County Commissioner Bruce Castor and former State Rep. Melissa Murphy Weber.
Greenleaf signed up to be on the presidential ballot for the Republican Party's New Hampshire primary. He explained that he did so to focus the debate of the election on the balancing of the federal budget. He filed with the FEC on December 29, and received a total of 24 votes in the primary, 21st place amongst ballot candidates. He won four write-in votes in the Democratic primary, all of which he received in Canaan. Including other write-ins, this tied him with Mitt Romney for third place in the town, behind only Barack Obama and Ron Paul.
