 Garnett was one of the seven children of Daniel F. Garnett and Hannah B. McDuffin Garnett.Her parents were among the earliest African-American settlers in Evanston, settling in the community known as "Ridgeland Village" and co-founding the Second Baptist Church of Evanston.Garnett did not graduate from high school, but did take classes in the 1890s at a business school in Minneapolis; beginning in 1894, she worked and studied at Chicago's Provident Hospital, the first black-owned hospital in the United States, established only a few years before.