Le Marchant was born at Newcastle-upon-Tyne, yet his family’s origins lay in Guernsey, where his lineal ancestor had been chief magistrate and lieutenant governor in 1204. His father, one of the most distinguished officers of the Peninsular War, was the founder of the Royal Military Academy, Sandhurst, and the Army Staff College, and fell leading the heavy brigade at Salamanca on 22 July 1812. His elder brother, Carey, was killed at Le Nive in November 1814.
Having for some time been active in Whig party management, Le Marchant was appointed joint secretary to the treasury in June 1841 and, having acquired an estate at Chobham, Surrey, was made a baronet by Lord Melbourne that August.
On 30 September 1850 Le Marchant was nominated chief clerk to the House of Commons at £2,000 per annum, a post he filled with great industry and only occasional lapses of duty.
