Joan Mervyn Hussey was educated at home in childhood, then at Trowbridge High School for Girls and at a lycée in Paris. She read history at St. Hugh's College, Oxford and obtained a PhD in 1935. She studied with Byzantine scholars in England and Germany and began investigating 11th-century manuscripts in the Hofbibliothek in Vienna, the Vatican Library, and in the monastery of St. Stephen on Meteora. In 1937, she was appointed assistant lecturer and later lecturer at the University of Manchester, then reader at Bedford College, London, and in 1950 she became professor of history at London University. She was named head of the History Department and served in that position until she retired in 1974.
Her doctoral thesis was expanded and published in 1937 as Church and Learning in the Byzantine Empire, 867-1185. In addition, she wrote many articles for scholarly journals on her interests in John Mauropous and Symeon the New Theologian, and in Byzantine monasticism.
At London University, J.M. Hussey introduced undergraduate courses on Byzantine topics. Her survey text on Byzantine life and history, The Byzantine World (1957), remains a model of its kind. For many years, she was engaged in editing and contributing to the Byzantine volumes in The Cambridge Medieval History. During her career, J.M. Hussey made many important contributions to Byzantine studies in Britain and worldwide, and was considered a formidable scholar with deep integrity and understanding of her subject.