Hays was born in 1920 in Buffalo, New York as the middle of three children in the family. Her father's name was Daniel Joseph McCabe II (1881–1939), while her mother's name was Mattie Florence Humphrey (1885–1961), who was of Welsh descent; both her parents were members of The Salvation Army. During Hays' childhood the family moved several times in the western New York and eastern Pennsylvania areas, but settled in Allentown, Lehigh County, Pennsylvania, in 1932. She had an elder brother, Daniel Joseph and a younger sister, Katherine Evangeline. Hays attended Allentown High School, now William Allen High School, graduating with honours in 1938. Hays had a love of music, playing the piano, the organ and the French horn, and wanted to go to Juilliard School to study music but due to a lack of funds for tuition she decided to pursue nursing instead.