He was born in London, the son of Thomas Frankland Lewis of Harpton Court, Radnorshire and his wife Harriet Cornewall. Thomas, after holding subordinate office in various administrations, became a poor-law commissioner, and was made a baronet in 1846.
His maternal grandparents were Sir George Cornewall, 2nd Baronet and Catherine Cornewall, daughter of Velters Cornewall.
Lewis was educated at Eton College and at Christ Church, Oxford, where in 1828 he earned a first-class in classics and a second-class in mathematics. He then entered the Middle Temple, and was called to the bar in 1831.
In 1833 he undertook his first public work as one of the commissioners to inquire into the condition of the poor Irish residents in the United Kingdom of Great Britain and Ireland. In 1834 Lord Althorp included him in the commission to inquire into the state of church property and church affairs in Ireland. To this fact we owe his work on Local Disturbances in Ireland, and the Irish Church Question (London, 1836), in which he condemned the existing connection between church and state, proposed a state provision for the Catholic clergy, and maintained the necessity of an efficient workhouse organization.
During this period Lewis's mind was occupied with the study of language. Before leaving college he had published some observations on Richard Whately's doctrine of the predicables, and soon afterward he assisted Connop Thirlwall and Julius Charles Hare in starting the Philological Museum. Its successor was the Classical Museum which he also supported "by occasional contributions.
In 1835 he published an Essay on the Origin and Formation of the Romance Languages (re-edited in 1862), the first effective criticism in United Kingdom of Great Britain and Ireland of François Juste Marie Raynouard's theory of a uniform romance tongue, represented by the poetry of the troubadours.
He also compiled a glossary of provincial words used in Herefordshire and the adjoining counties. But the most important work of this earlier period was one to which his logical and philological tastes contributed. The Remarks on the Use and Abuse of some Political Terms (London, 1832) may have been suggested by Jeremy Bentham's Book of Parliamentary Fallacies, but it shows all that power of clear, original thinking which marks his larger and later political works.
Moreover, he translated Philipp August Böckh's Public Economy of Athens and Muller's History of Greek Literature, and he assisted Henry Tufnell in the translation of Muller's Dorians. Some time afterward he edited a text of the Fables of Babrius.
While his friend Abraham Hayward conducted the Law Magazine, he wrote in it frequently on such subjects as secondary punishments and the penitentiary system. In 1836, at the request of Charles Grant, 1st Baron Glenelg, he accompanied John Austin to Malta, where they spent nearly two years reporting on the condition of the island and framing a new code of laws.
One leading object of both commissioners was to associate the Maltese in the responsible government of the island. On his return to Britain, Lewis succeeded his father as one of the principal poor-law commissioners. The Essay on the Government of Dependencies appeared in 1841, a systematic statement and discussion of the various relations in which colonies may stand towards the mother country.
In 1844 Lewis married Lady Maria Theresa Villiers, a lady of literary tastes. Much of their married life was spent in Kent House, Knightsbridge. They had no children.
Maria Theresa was the widow of novelist Thomas Henry Lister and mother of three children by her previous marriage. She was also a younger sister of George William Frederick Villiers, 4th Earl of Clarendon.
She was a daughter of George Villiers and Theresa Parker.
Jane Granville was a daughter of John Granville, 1st Earl of Bath and Jane Wyche.
In 1847 Lewis resigned his office. He was then returned for the county of Hereford, and Lord John Russell appointed him Secretary to the Board of Control, but a few months afterward he became Under-Secretary of State for Home Affairs. In this capacity he introduced two important bills, one for the abolition of turnpike trusts and the management of highways by a mixed county board, the other for the purpose of defining and regulating the law of parochial assessment.
In 1850 he succeeded Hayter as Financial Secretary to the Treasury. About this time, his Essay on the Influence of Authority in Matters of Opinion was published.
On the dissolution of parliament which followed the resignation of Lord John Russell's ministry in 1852, Lewis sought re-election in the United Kingdom general election, 1852. He was defeated for Herefordshire and then for Peterborough. Excluded from parliament he accepted the editorship of the Edinburgh Review, and remained editor until 1855.
During this period he served on the Oxford commission, and on the commission to inquire into the government of London. But its chief fruits were the Treatise on the Methods of Observation and Reasoning in Politics, and the Enquiry into the Credibility of the Early Roman History, in which he vigorously attacked the theory of epic lays and other theories on which Barthold Georg Niebuhr's reconstruction of that history was based.
In 1855 Lewis succeeded his father in the baronetcy. He was at once elected member for the Radnor boroughs, and Lord Palmerston made him Chancellor of the Exchequer. He had a war loan to contract and heavy additional taxation to impose, but his industry, method and clear vision carried him safely through.
After the change of ministry in 1859 Sir George became Home Secretary under Lord Palmerston, and in 1861, much against his wish, he succeeded Sidney Herbert (Lord Herbert of Lea) at the War Office. In that role he successfully argued against Russell's call for British mediation in the American Civil War in the autumn of 1862.
The closing years of his life were marked by increasing intellectual vigour. In 1859 he published the Essay on Foreign Jurisdiction and the Extradition of Criminals, a subject to which the attempt on Napoleon III's life, the discussions on the Conspiracy Bill, and the trial of Bernard, had drawn general attention. He advocated the extension of extradition treaties, and condemned the principal idea of Weltrechtsordnung which Mohl of Heidelberg had proposed.
His two latest works were the Survey of the Astronomy of the Ancients, in which, without professing any knowledge of Oriental languages, he applied a sceptical analysis to the ambitious Egyptology of Bunsen; and the Dialogue on the Best Form of Government, in which, under the name of Crito, the author points out to the supporters of the various systems that there is no one abstract government which is the best possible for all times and places. An essay on the Characteristics of Federal, National, Provincial and Municipal Government does not seem to have been published.
A marble bust of Sir George, by Weekes, stands in Westminster Abbey. A large monument was built in memory of Sir George Cornewall Lewis in the small village of New Radnor, Powys after he died and still stands today.
Lewis's large circle of friends included Sir E. Head, the Grotes, the Austins, Lord Stanhope, John Stuart Mill, Dean Milman, and the Duff Gordons. In public life he was described by Lord Aberdeen as notable "for candour, moderation, love of truth". His name has gone down in history as that of a many-sided man, sound in judgment, unselfish in political life, and abounding in good sense.