Magill was born in New York City, the daughter of
Roswell Magill and Katherine Deborah Biggins Magill. Her father was a tax lawyer and government official; her mother, also trained as a lawyer, was a
University of Chicago alumna. Catherine Magill attended the
Brearley School, earned her bachelor's degree at
Vassar College in 1946, and was a member of
Phi Beta Kappa. She completed doctoral studies in history at Yale University in 1954, with a dissertation titled "A Decade of Dissent in Germany: An Historical Study of the Society of Protestant Friends and the German Catholic Church, 1840-1848". Her dissertation advisor was
Hajo Holborn. == Career ==