O'Neill was also elected to represent Antrim in the Northern Ireland House of Commons in 1921 and served as its first Speaker, before standing down from his seat in 1929.

He sat on the Privy Council of Ireland, and was the sole surviving member of that body immediately prior to his death in 1982.

From 1939 to 1940, he was the Parliamentary Under-Secretary of State for India and Burma, and was the Lord Lieutenant of Antrim from 1949 to 1959.

O'Neill retired from the Westminster Parliament in 1952, having become the Father of the House the previous year, and was raised to the peerage as Baron Rathcavan, of The Braid in the County of Antrim, on 11 February 1953.

[3] Lord Rathcavan died in 1982 at the age of 99 and was succeeded by his eldest surviving son, Phelim.