Earl Fitzwilliam was a title in both the Peerage of Ireland and the Peerage of Great Britain held by the head of the Fitzwilliam family. The Fitzwilliams acquired extensive holdings in South Yorkshire, largely through strategic alliances through marriage. In 1410, Sir John Fitzwilliam of Sprotborough, who died in 1421, married Margaret Clarell, daughter of Thomas Clarell of Aldwark, the descendant of a major Norman landholding family. Thus did the Fitzwilliams acquire the Clarell holdings.
Sir William Fitzwilliam (c.1460–1534) was an Alderman and Sheriff of London and acquired the Milton Hall estate in Peterborough in 1502. His grandson Sir William FitzWilliam served as Lord Deputy of Ireland from 1571 to 1575 and from 1588 to 1594. His grandson William FitzWilliam was raised to the Peerage of Ireland as Lord Fitzwilliam, Baron of Liffer, alias Lifford, in the County of Donegal, in 1620. His grandson, the third Baron (who succeeded his father in 1658), was in 1716 created Earl Fitzwilliam, of the County of Tyrone with the subsidiary title Viscount Miltown, in the County of Westmeath, also in the Peerage of Ireland. He was succeeded by his son, the second Earl. He sat as Member of Parliament for Peterborough. On his death the titles passed to his son, the third Earl. He also represented Peterborough in the House of Commons. In April 1742 he was created Lord Fitzwilliam, Baron of Milton in the County of Northampton, in the Peerage of Great Britain, and in 1746 he was further honoured when he was made Earl Fitzwilliam, of Norborough with the subsidiary title Viscount Milton, both in the County of Northampton, also in the Peerage of Great Britain. Lord Fitzwilliam married Lady Anne Watson-Wentworth (d. 1769), daughter of Thomas Watson-Wentworth, 1st Marquess of Rockingham, and sister of Charles Watson-Wentworth, 2nd Marquess of Rockingham.
He was succeeded by his son, the fourth Earl. He was a prominent Whig politician and served as Lord President of the Council and as Lord-Lieutenant of Ireland. In 1782 he inherited the Watson-Wentworth estates (including Wentworth Woodhouse) on the death of his uncle Lord Rockingham, which made him one of the greatest landowners in the country. When he died the titles passed to his son, the fifth Earl. He represented several constituencies in the House of Commons and was made a Knight of the Garter in 1851. In 1856 Lord Fitzwilliam assumed by Royal license the additional surname of Wentworth. He was succeeded by his second but eldest surviving son, the sixth Earl. He sat as Member of Parliament for Malton and County Wicklow and served as Lord Lieutenant of the West Riding of Yorkshire. His eldest son William FitzWilliam, Viscount Milton, was also a member of parliament but predeceased his father. Lord Fitzwilliam was therefore succeeded by his grandson, the seventh Earl. He was the eldest son of Viscount Milton. He represented Wakefield in Parliament as a Liberal Unionist. When he died the titles passed to his son, the eighth Earl. He was killed in an aircrash in France in 1948.
On his early death the line of the eldest son of the sixth Earl failed and titles passed to the late Earl's first cousin once removed, the ninth Earl. He was the son of Captain the Hon. Sir William Charles Wentworth-Fitzwilliam, fourth son of the sixth Earl. When he died in 1952 this line of the family also failed and the titles were inherited by his second cousin, the tenth Earl William Thomas George Wentworth-FitzWilliam. He was the son of George Charles Wentworth-Fitzwilliam, son of the Hon. George Wentworth-FitzWilliam, MP, third son of the fifth Earl. On his death in 1979 all the titles became extinct. The family seat of Wentworth Woodhouse was sold while the more than 80,000 acre (320 km²) estate including much of the town of Malton, North Yorkshire, was retained. The other family seat, Milton Hall, and its considerable estate of over 50,000 acres (200 km²) together with valuable properties in Peterborough and the surrounding area continue by descent in the family. The Bourne Park Estate, near Canterbury, Kent, England, remains in the ownership of Lady Juliet Tadgell, née Wentworth-Fitzwilliam, only child of the 8th Earl by his wife the former Olive Dorothea Plunket.
The eldest son of the Earl Fitzwilliam bore the courtesy title Viscount Milton.
Barons Fitzwilliam (1620)
- William Fitzwilliam, 1st Baron Fitzwilliam (d.1644)
- William Fitzwilliam, 2nd Baron Fitzwilliam (c.1609–1658)
- William Fitzwilliam, 3rd Baron Fitzwilliam (1643–1719) (created Earl Fitzwilliam in 1716)
Earls Fitzwilliam (1716; 1746)
- William Fitzwilliam, 1st Earl Fitzwilliam (1643–1719)
- John Fitzwilliam, 2nd Earl Fitzwilliam (1681–1728)
- William Fitzwilliam, 3rd Earl Fitzwilliam (1719–1756) (created Earl Fitzwilliam in the Peerage of Great Britain in 1746)
- William Fitzwilliam, 4th Earl Fitzwilliam (1748–1833)
- Charles William Wentworth-Fitzwilliam, 5th Earl Fitzwilliam (1786–1857)
- William Charles Wentworth-FitzWilliam, Viscount Milton (1812–1835)
- William Thomas Spencer Wentworth-Fitzwilliam, 6th Earl Fitzwilliam (1815–1902)
- William Wentworth-FitzWilliam, Viscount Milton (1839–1877)
- William Charles de Meuron Wentworth-Fitzwilliam, 7th Earl Fitzwilliam (1872–1943)
- (William Henry Lawrence) Peter Wentworth-Fitzwilliam, 8th Earl Fitzwilliam (1910–1948)
- Eric Spencer Wentworth-Fitzwilliam, 9th Earl Fitzwilliam (1883–1952)
- William Thomas George Wentworth-FitzWilliam, 10th Earl Fitzwilliam (1904–1979)
- Brydges, Egerton Collins's Peerage of England; Genealogical, Biographical and Historical Vol. VIII (pp.374–400) London: F.C. and J. Rivington, 1812
- Marriage of Fitzwilliam and Clarell, Earls of Fitzwilliam, rotherhamweb
- By letters patent bearing the date at Westminster, 1 December 18 Jac. I
- By letters patent bearing the date at Westminster, 21 July 2 Geo. I
- By letters patent bearing the date at Westminster, 19 April 15 Geo. II
- By letters patent bearing the date at Westminster, 6 September 20 Geo. II
- The Annual Biography and Obituary Vol. XVIII (pp.93–106) London: Longman, Rees, Orme, Brown, Green and Longman, 1834