Bradley was born in 1882 in Chicago, Illinois. She graduated from Smith College in 1905 where she majored in English. After graduation she traveled to Egypt with a cousin and was inspired to write “The Palace of Darkened Windows” and “The Fortieth Door” detailing the life of the veiled and secluded women of Egypt. Both of these stories were later made into film further giving audience to Bradley’s writings. While doing research for her book “The Favor of Kings” in Oxford she met her husband Herbert Bradley. Herbert Bradley was a lawyer; big game hunter, traveler and explorer who later helped found the Brookfield Zoo. They were married in 1910 and five years later they had a daughter Alice.

In 1921 and 1922 Mary, Herbert and Alice traveled to the Belgian Congo with Carl E. Akley of the American Museum of Natural History, for specimens of the mountain gorilla for display in the museum. These expeditions were described in her books, “On the Gorilla Trail”, “Alice in Jungleland” and “Alice in Elephantland”. In 1938 her story "The Life of the Party" was chosen to appear in The The O. Henry Prize Stories anthology.. As a war correspondent for Colliers Magazine in 1945 Mary took on the difficult task of reporting on women in the military in Italy, France, and Germany. At the close of the war she recounted her tour of concentration camps in a magazine feature series on the Holocaust.

Bradley was a prolific author of mysteries, travel books, short fiction and novels most notably the Old Chicago series of historical novels . She was frequently asked to lecture on her travels and was inducted into the Society of Women Geographers, whose membership included Amelia Earhart, Margaret Mead, and Eleanor Roosevelt. Bradley was one of the few female presidents of the Society of Midland Authors as well as an active clubwoman in Chicago.

It was Bradley’s death in late October 1976 that inadvertently revealed that her daughter Alice B. Sheldon was prominent science fiction writer James Tiptree, Jr.