Bradford attended Harvard University briefly with the class of 1886, then continued his education with a private tutor, but is said to have been educated "mainly by ill-health and a vagrant imagination." As an adult, Bradford lived in Wellesley, Massachusetts. The building and student newspaper for the Wellesley High School (where Sylvia Plath received her secondary school education) were named after Gamaliel Bradford. The town changed the name of the building to Wellesley High School, but the newspaper maintains Bradford's name.
In his day Bradford was regarded as the "Dean of American Biographers." He is acknowledged as the American pioneer of the psychographic form of written biographies, after the style developed by Lytton Strachey. Despite suffering poor health during most of his life, Bradford wrote 114 biographies over a period of 20 years.