They lived at Exbury near Beaulieu, at the edge of the New Forest. Here, at Exbury House, his father John's property, Mitford was born. He was educated at Cheam School, under the picturesque writer William Gilpin, but at the age of fifteen a severe illness led to his being removed, and after two years of idleness Mitford was sent, in July 1761, as a gentleman commoner to Queen's College, Oxford. In this year his father died, and left him the Exbury property and a considerable fortune. Mitford, therefore, being "very much his own master, was easily led to prefer amusement to study." He left Oxford (where the only sign of assiduity he had shown was to attend the lectures of Blackstone) without a degree, in 1763, and proceeded to the Middle Temple. 
Historian of ancient Greece
He married Miss Fanny Molloy in 1766, the daughter of James Molloy of Dublin. He retired to Exbury for the rest of his life, and made the study of the Greek language his hobby and occupation. After ten years his wife died, and in October 1776 Mitford went abroad. He was encouraged by French scholars whom he met in Paris, Avignon and Nice to give himself systematically to the study of Greek history. But it was Edward Gibbon, with whom he was closely associated when they both were officers in the South Hampshire Militia, who suggested to Mitford the form which his work should take. In 1784 the first of the volumes of his History of Greece appeared, and the fifth and last of these quartos was published in 1810, after which the state of Mitford's eyesight and other physical infirmities, including a loss of memory, forbade his continuation of the enterprise, although he painfully revised successive new editions.
Later life and legacy
While his book was progressing, Mitford was a Tory member of the House of Commons, with intervals, from 1785 to 1818, but it does not appear that he ever visited Greece. A supporter of William Pitt the Younger, his patron was Hugh Percy, 1st Duke of Northumberland, owner of the pocket borough of Newport in Cornwall, which had 62 voters. This ended in 1790 but Mitford was assisted into one of the seats for Bere Alston in 1796 by his second cousin (and the duke's second son) Algernon Percy, 1st Earl of Beverley In 1812 he was elected to sit for New Romney in Kent, retiring in 1818. 
These links between Mitford and the dukes of Northumberland were continued by his grandson Henry's marriage to Beverley's granddaughter Lady Jemima Ashburnham in 1828.
In addition to his History of Greece, he published a few smaller works, the most important of which was an Essay on the Harmony of Language, 1774. The style of Mitford is natural and lucid, but without the rich colour of Gibbon. He affected some oddities both of language and of orthography, for which he was censured and which he endeavoured to revise.
Typical for such a man, Mitford was an impassioned anti-Jacobin from the 1790s, and his partiality for a monarchy led him to be unjust to the Athenians. Hence his History of Greece, after having had no peer in European literature for half a century, faded in interest on the appearance of the work of George Grote. Clinton, too, in his Fasti hellenici, charged Mitford with "a general negligence of dates," though admitting that in his philosophical range "he is far superior to any former writer" on Greek history. Byron, who dilated on Mitford's shortcomings, nevertheless declared that he was "perhaps the best of all modern historians altogether." This Mitford certainly was not, but his pre-eminence in the little school of English historians who succeeded Hume and Gibbon would be easier to maintain.
Mitford married, on 18 May 1766 Fanny Molloy (died 27 April 1827); they had three children:
- Capt Henry Reveley Mitford RN (1769–1804)
- John Mitford (1772-1851) barrister
- Bertram Mitford (1774-1844) barrister.
William Mitford's younger brother John (1748–1830) was a lawyer and politician who became Speaker of the House of Commons and Lord Chancellor of Ireland. Mitford's cousin, the Rev. John Mitford (1781–1859), was editor of the Gentleman's Magazine and of various editions of the English poets. He was distantly related to the novelist Mary Russell Mitford (1787–1865).
Mitford was the great-great-great-grandfather of the Mitford sisters, who came to public notice in Britain from the 1930s, as follows:
- Capt Henry Reveley Mitford RN (1769–1804) (see HMS York (1796))
- Henry Mitford (1804–84)
- Algernon, 1st Lord Redesdale (1837–1916)
- David, 2nd Lord Redesdale (1878–1958)
- The sisters
- Wroth 1894.
- Mitford's History of Greece, 1838 edition, can be read on google books. It includes a brief biography by his brother John, the 1st Lord Redesdale.
- The History of Parliament: the House of Commons 1790-1820, ed. R. Thorne, 1986
- Notes on Henry
- Wroth, Warwick William (1894). "Mitford, William". In Lee, Sidney. Dictionary of National Biography 38. London: Smith, Elder & Co.
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press
|Parliament of Great Britain|
John Riggs Miller
|Member of Parliament for Newport (Cornwall)
With: John Riggs Miller
Sir John Mitford
Sir George Beaumont
|Member of Parliament for Bere Alston
With: Sir John Mitford 1796–1799
Lord Lovaine 1799–1800
Parliament of the United Kingdom
|Parliament of the United Kingdom|
Parliament of Great Britain
|Member of Parliament for Bere Alston
With: Lord Lovaine
Hon. Josceline Percy
The Earl of Clonmell
Hon. George Ashburnham
|Member of Parliament for New Romney
With: Sir John Duckworth 1812–1817
Cholmeley Dering 1817–1818