[2] In 1801 he inherited the family estates in Queen's County on the death of his father,[1] bypassing his disabled elder brother according to a special Act of Parliament passed in 1789.

[1] The Whigs came to power under Lord Grey and in 1831 Parnell was admitted to the Privy Council[7] and appointed Secretary at War,[8] a post he held until February 1833.

[9] When the Whigs again came to power in April 1835 under Lord Melbourne, Parnell was made Paymaster of the Forces and Treasurer of the Ordnance and Navy.

[1] Parnell was opposed to the prevailing protectionist system and advocated retrenchment of public expenditure, especially for the armed services.

These ideas were defended by Parnell and others in opposition to those of the British Currency School who advocated legal restrictions on the amount of notes that could be issued, with respect to their deposits in specie, and the British Banking School, which advocated discretionary policy by the banks in monetary matters.

In the domain of civil engineering, he authored the 1833 and 1838 editions of A Treatise on Roads, in which the works and techniques of Thomas Telford were described.

[1] Parnell married Lady Caroline Elizabeth, daughter of John Dawson, 1st Earl of Portarlington, in 1801.

His great grandson Henry Parnell, 5th Baron Congleton was the youngest Member of Parliament killed in World War I.