Cahalan was one of seven children born to Michael and Catherine Cahalan, both natives of Ireland. During the Civil War the family lived in Columbus, Georgia, and young Mary was trained at St Joseph's Convent there. She and her family moved to the new city of Birmingham in 1872, residing at the Relay House until their three-story home, the first brick residence in the city, was completed at 4th Avenue and 21st Street. It was there that the first Mass was conducted in the young city, and Father William McDonough resided with them and helped to care for the sick during the 1873 cholera epidemic.
She was appointed principal of the Free School in 1885, and held the job until her death.
Cahalan was an active member of the Alabama Educational Association and presented numerous papers in regional and national conferences. She was the first president of the St Paul's Biblical and Literary Society, founded in 1894.
- Cruikshank, George H. (1920) History of Birmingham and Its Environs: A Narrative Account of Their Historical Progress, Their People, and Their Principal Interests. 2 volumes. Chicago, Illinois: Lewis Publishing Company.
- Mary A. Cahalan at Findagrave.com