McKisack was born on 30 March 1900 in
Belfast, Northern Ireland, to Audley John McKisack, a solicitor, and Elizabeth McKisack (née McCullough). When her father died in 1906, her mother took May and her brother
Audley (1903–66) to live in
Bedford, England. She was educated at
Bedford High School, an all-girls
independent school. In 1919, she
matriculated at
Somerville College, Oxford, where her tutor in history was
Maude Clarke. She graduated with a
Bachelor of Arts (BA) degree, and then taught in a school for one year. She returned to Somerville where she was Mary Somerville
research fellow while she studied for the
postgraduate Bachelor of Letters (BLitt) degree. She was a
lecturer in
medieval history at the
University of Liverpool from 1927 to 1935, before returning to Somerville College, Oxford in 1936 as
fellow and
tutor. She was additionally a university lecturer at the
University of Oxford between 1945 and 1955. In 1955, she left Oxford having been appointed Professor of History at
Westfield College,
University of London. She was made an
honorary fellow of Somerville College in 1956. She retired in 1967, and was made Emeritus Professor of History by the University of London. McKisack was elected a
Fellow of the Royal Historical Society (FRHistS) in 1928 and as a
Fellow of the Society of Antiquaries of London (FSA) in 1952. ==Selected works==