In 1739, Ponsonby entered the Irish House of Commons for Newtownards, becoming its speaker in 1756. He also served as First Commissioner of the Revenue and he became a member of the Privy Council of Ireland in 1746. In 1761, Ponsonby was elected for Kilkenny County and Armagh Borough, and sat for the first. In 1768, he stood also for Gowran and Newtownards, and in 1776 for Carlow Borough, but chose each time Kilkenny County, which he represented until 1783. Subsequently Ponsonby was again returned for Newtownards and sat for this constituency until his death in 1787.
Belonging to one of the great families which at this time monopolized the government of Ireland, Ponsonby was one of the principal "undertakers," men who controlled the whole of the king's business in Ireland, and he retained the chief authority until George Townshend, 1st Marquess Townshend became lord-lieutenant in 1767. Then followed a struggle for supremacy between the Ponsonby faction and the party dependent on Townshend, one result of this being that Ponsonby resigned the speakership in 1771.