Earl of Haddington is a title in the Peerage of Scotland. It was created in 1627 for the noted Scottish lawyer and judge Thomas Hamilton, 1st Earl of Melrose. He was Lord President of the Court of Session from 1616 to 1625. Hamilton had already been created Lord Binning in 1613 and Lord Binning and Byres, in the County of Haddington, and Earl of Melrose, in the County of Roxburgh, in 1619. These titles were also in the Peerage of Scotland. The title of the earldom derived from the fact that he was in possession of much of the lands of the former Melrose Abbey. However, Hamilton was unhappy with this title and wished to replace it with "Haddington" (a title which was then held by John Ramsay, 1st Earl of Holderness and 1st Viscount of Haddington, but on whose death in 1626 both peerages became extinct). In 1627 he relinquished the earldom of Melrose and was instead created Earl of Haddington, with the precedence of 1619 and with limitation to his heirs male bearing the surname of Hamilton. This derived from the fact that he considered it a greater honour to take his title from a county rather than from an abbey. Hamilton was a member of the prominent Scottish family of that name and descended from John de Hamilton, younger son of Walter de Hamilton (or Walter Fitzgilbert), who was granted the feudal barony of Cadzow and who is also the ancestor of the Dukes of Hamilton and Dukes of Abercorn.
Lord Haddington was succeeded by his eldest son, the second Earl. He was a staunch Covenanter. Haddington served as Governor of the Castle of Dunglass, and was killed by a massive explosion there in 1640. His eldest son, the third Earl, died childless at an early age and was succeeded by his younger brother, the fourth Earl. On his death the titles passed to his son, the fifth Earl. He married Margaret Leslie, 8th Countess of Rothes (d. 1700), daughter of the noted statesman John Leslie, 1st Duke of Rothes, who had received a re-grant of the earldom of Rothes in 1663 which allowed it to be passed on to his daughter (see the Earl of Rothes for earlier history of this title). According to the regrant of 1663, the earldom of Rothes was not allowed to be united with the earldom of Haddington. The couple were therefore in 1689 granted a patent of the marriage contract, which stated that the earldom of Rothes should descend to their eldest son, the Hon. John, while the earldom of Haddington should be inherited by their second son, the Hon. Thomas. According to this patent Lady Rothes was succeeded by her eldest son John, the ninth Earl (who assumed the surname of Leslie; see the Earl of Rothes for further history of this branch of the family).