From NKAA, Notable Kentucky African Americans Database (main entry)
In 1921, Henry King, aboard the horse Planet, was the last African American jockey to ride in the Kentucky Derby until the year 2000 when jockey Marlon St. Julien rode in the Kentucky Derby. Henry King finished 10th in the 1921 Derby. Source: Black Winning Jockeys in the Kentucky Derby by J. R. Saunders and M.R. Saunders.