Callahan was born on the Peepeekisis Cree Nation in Saskatchewan to Nora and John Thomas, and was granted the spirit name "Wapiskisiw Piyésís" (meaning White Birdwoman) by an elder at age four.
[3] Callahan was one of the first Indigenous graduates of the Winnipeg General Hospital's nursing school, convocating in 1958.
[1] She was head nurse of a gynecology ward before joining a new organization, Continuing Care for People in Need, founded in 1973 to support the health needs of those in Winnipeg's core.
[1] After retirement she attended university, achieving a bachelor's degree focused on psychology and a masters in interdisciplinary studies.
[8] She was also involved in the creation of the Registered Nurses of Canadian Indian Ancestry, now called the Canadian Indigenous Nurses Association, considered the country's first professional organization for Indigenous peoples.