Robert Sengstacke Abbott was born on November 28, 1868 at Frederica, St. Simon's Island, Georgia, United States. He was the first son and second child of former slave parents, Thomas and Flora (Butler) Abbott. His father died shortly after the boy was born, and when he was five years old his mother married John Hermann Henry Sengstacke, the owner of a general store in Savannah, who was the half-white son of a slave mother and a German father. She bore him seven children.
From his stepfather, who subsequently became a schoolteacher and Congregational clergyman, young Abbott developed a passion for the cause of Negro citizenship.