Born Ann Gordon on November 18, 1895 in Verdun, Montreal, Quebec, she married Montréal businessman William O'Brien and led an active social life combined with volunteer work at the Sisters of Service Hostel for Immigrant Women in Montréal. In 1941, after her husband's death, she entered the Sisters of Service in Toronto. In 1946 she was asked to be executive director of the newly formed Catholic Welfare Bureau, later called Catholic Family Services, in Saskatoon. Sister O'Brien remained executive director until her retirement in 1976. During her thirty years of service in Saskatoon, her work was officially recognized by many agencies and organizations; she was named Saskatoon's Citizen of the Year in 1974. On her retirement in 1976, the Board of Catholic Family Services and other public-spirited citizens established “The Sister Ann O'Brien Scholarship Fund” to commemorate her service and dedication to family service work. In 1980 she was named among the “Notable Saskatchewan Women: 1905-1980” and in 1982 Sister O'Brien School and O'Brien Crescent, Terrace & Court in Saskatoon were named in recognition of her work.
Ann O'Brien's gifts and skill as a social worker and family counselor are legendary. Her openness to all people regardless of class or creed, her dedication, concern, wisdom, advocacy and witness to Christian values were acclaimed and esteemed by all who knew her. She died at the SOS Retirement Centre in St. Catharines, Ontario, on April 12, 1986.