Upon graduation from law school, he became a founding partner of the law firm of Leeson, Leeson & Leeson in Bethlehem, Pennsylvania. The firm conducted a general civil practice with an emphasis on litigation. Leeson earned certification as a civil trial advocate from the National Board of Trial Advocacy and also served as an arbitrator for the American Arbitration Association. During his career, he served as a solicitor for several municipalities in Pennsylvania.[2][3] In 2017 he was awarded an honorary Doctor of Law degree by DeSales University.[citation needed]
Leeson previously held elected public office as a member of the city council in Bethlehem, where he served as vice president and chair of the finance committee. He also was elected as a member of the Northampton County Government Study Commission where he co-authored the Home Rule Charter for Northampton County.
He has served as chair of the boards of the Lehigh Valley Public Telecommunications Corporation, PBS 39, the Lehigh Valley Community Foundation, the Lehigh–Northampton Airport Authority, and the Janet Johnston Housenick and William D. Housenick Memorial Foundation. He formerly served as a member of the board of commissioners of the Pennsylvania Public Television Network.