As his career is described in the D.N.B. this account can be brief. He lived at various times in Europe, where he held Government posts, e.g. in Paris (1700 or 1701). In June 1702 he was at Carmarthen, possibly as schoolmaster. He became secretary to Robert Harley (afterwards earl of Oxford) in 1704, whilst in 1708 he was a British Government secretary in Brussels; later he was under-secretary of State under the earl of Dartmouth, etc. From 1710 Dean Swift in his Journal to Stella makes frequent references to Lewis, who had become a member of a literary and political circle which included the Dean, Robert Harley, Alexander Pope, John Arbuthnot, and a person who is not named in that connection in D.N.B., viz. Thomas Mansel (1st baron Mansel) of Margam, Glam. (q.v.). There are several letters, 1700-13, from Lewis to Thomas Mansel in the Margam and Penrice collection in the N.L.W. — letters which are full of political, military, and social news (and gossip) and which throw interesting light on the history of Britain and Europe; in one letter (16 Sept. 1704) Lewis says that he hoped to try to buy ‘the Estate late Sir Rice Rudds, in Carmarthenshire.’ In Oct. 1712 he was given the post of provost-marshall-general in the Barbadoes, but he appears to have discharged the duties of the post by deputy; in Nov. of the same year he was returned M.P. for Lostwithiel, Cornwall.

Lewis m., 1 Oct. 1724 (at S. Benet's, Paul's Wharf, London), Anne Bateman (née Jennings), widow of Thomas Bateman; they lived in Cork Street, Burlington Gardens, London. He d. 10 Jan. 1754 and was buried in Westminster abbey; his wife had been buried there previously (25 Nov. 1736). He left property in various Welsh parishes.