Harry P. O'Neill was born in Dunmore, Pennsylvania. He left school at the age of ten and went to work as a slate picker in the O.S. Johnson Colliery in Dunmore. He worked evenings as an apprentice barber until the age of sixteen and at the age of eighteen purchased his employer’s business. He was also engaged as an insurance broker. He served in the Pennsylvania State House of Representatives from 1929 to 1948.
O'Neill was elected as a Democrat to the Eighty-first and Eighty-second Congresses, but he was an unsuccessful candidate for reelection in 1952. He was survived by his grandson Francis M. O'Neill and his great grandchildren Harry P. O'Neill and Abby E. O'Neill.