Born in Somerville, Massachusetts, on May 20, 1862, Mabel Keyes Babcock was the daughter of botanist Henry H. Babcock and Mary Porter (Keyes) Babcock. She was a descendant of William Bradford, the governor of Plymouth Colony. Both of her parents were involved in education: Henry was for a time the principal of Somerville High School in Massachusetts, while Mary, after Henry died, became the headmistress of Kenilworth Hall, a girls' school in the Chicago area.