Vernon was the son of Robert Percy Smith, of 20 Savile Row, London, and of Cheam, Surrey, and the nephew of The Rev. Sydney Smith, Canon of St Paul's. His mother was Carolina Maria Vernon, daughter of Richard Vernon. Vernon was educated at Christ Church, Oxford (2nd class classics 1822).
He was elected Member of Parliament for Tralee in 1829, a seat he held until 1831, and then sat for Northampton from 1831 to 1859. When the Whigs came to power in 1830 under Lord Grey, Vernon was appointed a Lord of the Treasury (government whip), which he remained also when Lord Melbourne became Prime Minister in July 1834. The Whigs fell from office in November of that year, but returned already in April 1835, when Vernon was appointed Secretary of the Board of Control by Melbourne, which he remained until 1839. He then served as Under-Secretary of State for the Colonies from 1839 to 1841. The latter year he was also admitted to the Privy Council. He did not hold office again until February 1852, when he was made Secretary at War in the first administration of Lord John Russell. However, the government fell already the same month. When the Liberals (as the Whigs were now known) returned to office in 1855 under Lord Palmerston, Vernon was appointed President of the Board of Control, with a seat in the cabinet, a post he retained until the government fell in March 1858. The following year he was raised to the peerage as Baron Lyveden, of Lyveden in the County of Northampton.
Lord Lyveden married Emma Mary Fitzpatrick, daughter and co-heir of the Earl of Upper Ossory, in 1823. In 1846 he assumed for his children by Royal license the surname of Vernon in lieu of Smith and in 1859 he assumed for himself by Royal license the same surname in lieu of Smith. Lord Lyveden died in November 1873, aged 73, and was succeeded in the barony by his son Fitzpatrick Henry Vernon.