Born into a prestigious planting family of Irish Catholic background, she was educated at the Villa Maria Convent in Quebec and the Atlanta Female Seminary in Georgia.

Mitchell was born Mary Isabel Stephens on January 13, 1872, at her parents' Jackson Street mansion in Atlanta.

[1][2] She was the seventh child of Captain John Stephens, a Confederate officer and merchant who served on the Atlanta Police Commission, and Annie Fitzgerald, a landowner.

[2] Her mother was the daughter of planter Philip Fitzgerald, an Irish Catholic emigrant who owned Rural Home, a plantation in Clayton County near Jonesboro.

[2] Mitchell attended a finishing school at the Villa Maria Convent in Quebec, where she learned to speak French fluently, and graduated with honors from the Atlanta Female Seminary in 1892.