Fuller was born in Liberia, the son of a previously enslaved African who had purchased his freedom and emigrated there. Fuller graduated with an MD in 1897 from Boston University School of Medicine, which as a homeopathic institution was open to both African-American and women students. He later pursued further research at the psychiatric clinic of the University of Munich, Germany.
Most of his career, Fuller spent it practicing at Westborough State Mental Hospital in Westborough, Massachusetts. During his time there, he performed his ground-breaking research on the physical changes to the brains of Alzheimer’s patients.
Fuller was a member of APA early in his career, dropped out for a period, and in 1942 applied for reinstatement as a fellow, which was granted. When the Veterans Administration opened the Tuskegee Veterans Administration after WWI with an entirely black staff, he was instrumental in hiring and training black psychiatrists for key positions.