Ynes was the only child of Enrique Antonio and Sarah R. (wilmer) Mexia. Her father was on the diplomatic mission in Washington D.C for the Mexican government. Her fatherly grandfather, General Jose Antonio Mexia, was a leader of Mexico's Federalist Party and remarkable participant in early history of Texas. Through her mother, Mexia was related to Samuel Eccelston, the Roman Catholic archbishop of Baltimore. Ynes spent childhood on her family's grange near present-day Mexia in Limestone County, Texas. Her childhood was affected greatly by her parents marital problems. Mexia lived in Mexico City on her father's hacienda from her late teens into her late thirties. By the time she was sixteen, her family had moved to Philadelphia, where Ynes attended private Quaker school. Later she registered for a Quaker-sponsored school in Ontario, Canada, as well as Saint Joseph's Academy in Emmitsburg, Maryland. She attended The University of California intermittently from 1921 until 1938.