Cass began his career as a bookseller at The Economist Bookshop in
Bloomsbury, central London. He first published books of history and the social sciences whose copyright had expired. He later published new research, including biographies and military histories. By the late 1960s, he purchased the Woburn Press, a publishing house of works of literature. He also started publishing academic journals, beginning with
Middle Eastern Studies in 1964. He later published
Business History,
The Journal of Peasant Studies,
The Journal of Commonwealth and Imperial History,
West European Politics,
Slavery and Abolition,
Immigrants and Minorities,
Intelligence and National Security,
Jewish Culture and History and
Holocaust Studies. Cass purchased
Vallentine Mitchell, an imprint of Jewish books, in 1971. In 1972, he purchased the scripts of
The Goon Show. In 1974, he purchased the Irish Academic Press, which included the 1,000 volumes of British Parliamentary Papers from 1801 to 1901. Cass made much of his wealth through the Irish Academic Press. He also founded Vallentine Mitchell Academic in 2003. That year, Cass sold most of his journals to
Taylor & Francis for £15 million. However, he retained ownership of
Jewish Culture and History and
Holocaust Studies. ==Personal life and death==