Isabella Dinsmore Selmes was born the daughter of Tilden Russell Selmes (1853–1895) and Martha "Patty" Macomb Flandrau (1861–1923). Isabella was born at the historic
Dinsmore Farm in
Boone County, Kentucky which was owned by her mother's maternal great aunt
Julia Stockton Dinsmore (1833–1926). Her father Tilden Selmes was a Yale-educated attorney who originally practiced in St. Paul where he met her mother. Her mother Martha "Patty" Flandrau was the daughter of Minnesota Supreme Court judge and politician
Charles Eugene Flandrau (1828–1903) and his first wife Isabella Ramsay Dinsmore (1830–1867). The Selmes family owned a ranch 15 miles west of Mandan in
Dakota Territory and was on the same rail line as
Theodore Roosevelt's ranches in Medora 150 west of town. Her father Tilden and Theodore met in St. Paul while both were waiting their west-bound train. The Selmes family hosted him multiple times at their ranch and developed a close friendship with each other. to be near Patty's family. Tilden continued to practice law, and was for a time an associate counsel for the
Northern Pacific Railroad. After the untimely death of her father in 1895, Isabella and her mother lived with various members of her mother's family in Kentucky, Minnesota, and New York. Patty supported them by selling bacon and ham and working as a chaperone. where she met and became lifelong friends with Roosevelt's niece,
Eleanor. Isabella finished school in 1904, but did not graduate. As Patty had a drinking problem, and with a smaller inheritance from Flandrau than expected, Isabella's debut was seen by the family as a way to not only secure her future but also "keep her mother from succumbing to drink and despair." Isabella was successful in society. She became friends with Eleanor Roosevelt's cousin,
Corinne Robinsion, who would read a
Jack London book to Isabella as they drove to balls to make sure they remembered the world's problems. ==First and second marriages==