Fairfax Harrison was born on March 13, 1869 in New York City. He was the oldest of the three sons of Burton Norvell Harrison and Constance (Cary) Harrison. He was christened Reginald Fairfax, though he never used his first name. His father, a member of an old Virginia family, served the Confederacy as private secretary to Jefferson Davis and, after the war, moved to New York City, where he practised law. Harrison's mother was also a Virginian, a descendant on her mother's side of the Fairfax family; during the late nineteenth century she was a popular writer of novels and other fiction.