Parsons sat then as Member of Parliament (MP) for King's County from 1791 until the Act of Union in 1801.

In the following co-option, he chose to sit for King's County also in the British House of Commons, a seat he held until 1807.

In 1793 the Parsonstown Loyal Independent Volunteers raised by his family in 1776 as part of the Irish Volunteers was taken onto the official Irish Militia establishment as the King's County Royal Rifle Militia, and he served as the regiment's Colonel until 1798.

[1] In 1809, he became one of the Postmasters General of Ireland[2] with Charles O'Neill, 1st Earl O'Neill, with whom he attended the laying of the foundation stone for the new General Post Office in Dublin on 12 August 1814 by the Lord Lieutenant of Ireland, Charles Whitworth, 1st Earl Whitworth.

[3] He later sat in the House of Lords as an Irish representative peer from 1809 until 1841 and served as Custos Rotulorum of King's County from 1828 until his death.