Marsh was the principal of a design firm originally known as Neville Marsh Interiors (1965, dates vary), later becoming one of the principals of Marsh Freedman, in partnership with George Freeman (ca.1975, dates vary). Specialising in residential interiors, Marsh, formerly with the British Colour Council, was known for his colour work. He retired in 1986 and moved to Italy, later reviving his career with hotel work in Bali.