Posey joined the Homestead Grays as an outfielder in 1910, but his great success in black baseball came as a result of his managerial and business acumen. In 1916 he was named the Homestead manager. He fielded consistently talented, uncompromising teams that played high-caliber baseball. He possessed a great baseball mind and was a literate and dictatorial manager.
By the early 1920s, Posey owned the Grays, and he turned the team into a profitable business venture. After taking leadership roles in the organized black baseball leagues, he returned to run the Grays. Homestead continued to be a barnstorming power and eventually became the dominant team in the second Negro National League. Posey remained one of the most powerful men in black baseball until his death.