[2] He was educated at the Salesian College, Battersea, and trained as an actor at RADA, graduating in 1963,[3] before appearing at the Bristol Old Vic and in repertory theatre and in the 1986 first national tour of The Sound of Music as Captain von Trapp.

His other film credits include Rasputin the Mad Monk (1966), Alfie (1966), The Witches (1966), The Viking Queen (1967), Quatermass and the Pit (1967),[4] Mosquito Squadron (1969), I Start Counting (1970), Man in the Wilderness (1971), Because of the Cats (1973), The Tamarind Seed (1974) and The Spy Who Loved Me (1977).

[5] His later film career included roles in Australian productions such as BMX Bandits (1983), Bliss (1985), The Man from Snowy River II (1988), The Punisher (1989) with Dolph Lundgren, Country Life (1994) and Selkie (2000).

Marshall had several leading roles on television, notably Dobbin in the 1967 production of Vanity Fair, Gilbert Markham in the 1968 serial of The Tenant of Wildfell Hall and Wentworth in the 1971 adaptation of Persuasion.

He also worked extensively in Australia,[7] with credits including Prisoner,[nb 1] Special Squad, Golden Pennies, Neighbours, Embassy, Home and Away, Stingers, Water Rats and All Saints.