dewar was born marion bell in 1928 in montreal, the daughter of agnes and wilson bell. she was raised in the town of buckingham, quebec, just outside ottawa. she graduated from saint joseph's school of nursing in kingston, ontario, in 1949 and was a nurse in the ottawa region until 1952. she married civil servant ken dewar in 1951 and went into public health with the victorian order of nurses. a devoted roman catholic, she would have 5 children, the last in 1963. she later studied nursing science and public health at the university of ottawa, and was a public health nurse from 1969 to 1971.