Born in Brixham, Devon, in 1907, Nancy Breary was the eldest daughter of bank manager Arthur Henry Breary, and his wife, Edith Florence. Her younger sister, Gertrude, known as Gretchen, was born in 1908, and her younger brother, Gerald, in 1913. The family moved to London when Breary was still a baby, living in Clapham Park, and then Streatham. She was educated at Kingsdown School in Dorking, where she was sent as a boarder, from 1918 to 1924.
Intending to become a dietician, Breary took a domestic science course upon leaving school, but ended up working as a mannequin in a court dressmaker’s shop instead, while also running the family home, after her mother became an invalid. Breary wrote in her spare time at first, eventually switching over to writing as a full-time occupation in 1943, following the publication of Give a Form a Bad Name. Her sister Gretchen, who worked as an illustrator (her work was credited to "G.E. Breary"), was the primary breadwinner in the family, during this period. The Brearys lived briefly in Canada, in the 1950s, before returning to Great Britain in 1955, settling first in Rye, and then Winchelsea. Nancy Breay died, in Winchelsea, in 1988.
Breary was a prolific contributor to the Girls' School Story genre, publishing her first novel, Give a Form a Bad Name, in 1943, and going on to author twenty-six more, concluding with the 1962 Too Many Girls. Although her work does address some of the realities of the school experience - the jealousies amongst the girls, the unpleasantness of being new, and an outsider, the conflict between duty and pleasure - it is rarely intended to be a depiction of "normal life" in a boarding school. Rather, her books offer amusing stories - sometimes almost parodies - complete with many over-the-top elements, from schoolgirl feuds, to secret societies.