Bassett was born in Melbourne to academic parents, Sir David Orme Masson, a professor of chemistry, and his wife Mary, née Struthers. She grew up in and around the University of Melbourne. Bassett received most of her education at home from a governesses, although when she was 17 for three months she attended a girls' grammar school run by the Church of England. She became her father's secretary, but managed to attend history lectures at the university, particularly those of Ernest Scott. Scott encouraged her historical studies, and in 1913 she published a note in the University Review on the founding of the University of Melbourne.
During World War I she was a secretary to various military hospital directors, and was transferred to England in 1916. On the way, her ship, the Arabia was torpedoed and she was forced to escape in a life boat. After the war she returned home and met and married Walter E. Bassett, an engineer. They were married on 25 January 1923, and had several children. As her children grew older, Bassett began to write in earnest.
The Governor's Lady (1940) was a study of Philip Gidley King's wife, Anna Josepha King, the first woman to come to Australia as a governor's wife. This was a very early entry in the field that became Women's history. This was followed by The Hentys (1954), an account of the Henty family's trials along the Swan River, in Van Diemen's Land and finally in the Port Phillip District now in Victoria. This book has been praised as "combining diligent research, intelligent handling of historical evidence, and a prose style that is clear and harmonious." Bassett wrote many articles for the Australian Dictionary of Biography.
- 1968 Honorary Doctorate in Literature degree from Monash University
- 1969 foundation fellow of the Australian Academy of the Humanities
- 1971 Fellow of the Royal Historical Society of Victoria
- 1974 Honorary Doctorate in Literature degree from the University of Melbourne
- Wilde, William H. et al. (eds.) (2005) "Bassett, Marnie" The Oxford Companion to Australian Literature Oxford University Press, ISBN 0-19-553381-X
- Blainey, Ann (1993) "Bassett, Lady Flora Marjorie (Marnie) (1889-1980)" Australian Dictionary of Biography Volume 13, Melbourne University Press, p. 128
- Blainey, Ann (1993) "Bassett, Lady Flora Marjorie (Marnie) (1889-1980)" Australian Dictionary of Biography Volume 13, Melbourne University Press, pp 127–128.
- Wilde, William H.; Hooton, Joy W. and Andrews, B. G. (eds.) (1994) "Bassett, Marnie" The Oxford Companion to Australian Literature Oxford University Press, OCLC 32470151
- Bassett, Flora Marjorie in The Encyclopedia of Women and Leadership in Twentieth-Century Australia