Born in Lismore, Marsden attended a Catholic school in Cabramatta, St John's Preparatory College in Campbelltown and St Joseph's College, Hunters Hill, before entering a seminary to study for the priesthood. After deciding the priesthood was not for him, he completed a law degree at the University of Sydney, graduating in 1966 with a Bachelor of Laws. In 1968 Marsden established Marsdens, a law firm in Campbelltown. The firm grew from Marsden as a sole practitioner to being among the largest law firms in the Sydney area with 140 personnel and offices in Sydney, Liverpool, Camden, and Campbelltown. Marsden was a prominent resident of Campbelltown throughout his life and was often quoted as saying "There are two great cities in the world - Rome and Campbelltown." In 1974, Marsden was awarded a Master of Laws.