Thomas O'Neill was a member of the Wisconsin State Assembly.
O'Neill was born on June 21, 1821 in Dublin, Ireland. He settled in what is now the City of Greenfield, Wisconsin in 1847 . On February 12, 1874, in Milwaukee, he married Mary Reynolds, in a Catholic ceremony at St Gall's Church.
O'Neill was a member of the Wisconsin State Assembly during the 1875 session.