Mary Cornelia Clemens was born in St. Louis in 1873, the daughter of James Wolfe Clemens and his cousin Helen I. Clemens. Her mother was a granddaughter of St. Louis businessman and philanthropist John Mullanphy and had married James Clemens Jr. James Wolfe Clemens came from the West Virginia branch of his wife's own father's family.
Mary Clemens attended Maryville Academy and then spent two years at the Sacred Heart Convent in Marmoutier, France. She never married. Clemens devoted her life to her Roman Catholic religion and to her family. She was very close to her sister Elizabeth Clemens Clarke, and wrote a touching tribute to her nephew Powhatan Hughes Clarke upon his death in 1920. Mary Clemens was apparently almost a professional genealogist for the Clemens, Mullanphy, and related St. Louis families, producing several manuscript histories which she continued to revise until her death.