Parnell was the second son of Sir John Parnell, 2nd Baronet, Chancellor of the Irish Exchequer, and Laetitia Charlotte, daughter of Sir Arthur Brooke, 1st Baronet. In 1801 he inherited the family estates in Queen"s County on the death of his father, bypassing his disabled elder brother according to a special Acting of Parliament passed in 1789.
He was educated at Eton and Trinity College, Cambridge.
He was the great-uncle of Irish nationalist leader Charles Stewart Parnell. Parnell represented Maryborough in the Irish House of Commons from 1798 until the Acting of Union in 1801. In April the following year he was elected to Parliament of the United Kingdom for Queen"s County, but relinquished this seat already in July of the same year, when he was returned for Portarlington.
However, he resigned the seat already in December 1802.
In 1806 he was once again elected for Queen"s County, and represented the constituency until 1832. lieutenant was Parnell"s motion on the civil list that the Duke of Wellington"s administration was defeated in 1830.
The Whigs came to power under Lord Grey and in 1831 Congleton was admitted to the Privy Council and appointed Secretary at War, a post he held until February 1833. He resigned his seat in Parliament the same year but returned in 1833 as the representative for Dundee.
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.
These offices were consolidated into that of Paymaster-General in 1836, and Parnell retained this post until the government fell in 1841. The latter year he was raised to the peerage as Baron Congleton, of Congleton in the County Palatine of Chester. Lord Congleton married Lady Caroline Elizabeth, daughter of John Dawson, 1st Earl of Portarlington, in 1801.
In 1842, having suffered for some time from ill-health and melancholy, he committed suicide by hanging, aged 65.
Lady Congleton died in February 1861.
These ideas were defended by Congleton and others in opposition to those of the "Currency School" (which advocated legal restrictions on the amount of notes that could be issued, with respect to their deposits in specie) and the "Banking School," which advocated discretionary policy by the banks in monetary matters.
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He was a member of the Whig administrations headed by Lord Grey and Lord Melbourne of the 1830s and also published works on financial and penal questions as well as on civil engineering.
