John O'Hart (1824–1902) was an Irish genealogist. He was born in Crossmolina, Co. Mayo, Ireland. A committed Roman Catholic, O'Hart originally planned to become Catholic priest but instead spent 2 years as a police officer. He was an Associate in Arts at the Queen's University of Belfast. He worked at the Commissioners of National Education during the years of the Great Irish Famine. He worked as a genealogist and took an interest in Irish history. He was an Irish nationalist. He died in 1902 in Clontarf near Dublin, at the age of 78.