Marshall was a founder member of London Video Arts in 1976, and was a committed advocate of British video art, as a practitioner, curator and theorist. He curated the first UK/Canadian Video Exchange in 1984 and his videos and writings were amongst the first to explore the relationship between video, television and the media. With later works such as Bright Eyes, he explored, and challenged, misrepresentations of homosexuality during the AIDS epidemic of the 1980s, at a time when lesbian and gay lifestyles and sexuality were under attack as a result of Clause 28 and the media-encouraged prejudice surrounding the spread of AIDS. Towards the end of his life, working with Maya Vision, Marshall made a number of Channel 4 commissioned documentaries concerning gay identity and he continued to be a passionate campaigner for gay rights.