Kennedy was the eldest son of Archibald Kennedy, 11th Earl of Cassilis, by Anne, daughter of John Watts and descendant of the Schuyler family, the Van Cortlandt family (including Stephanus Van Cortlandt), and the Delancey family of British North America. He became known by the courtesy title Lord Kennedy when his father succeeded to the earldom of Cassilis in 1792.
Kennedy succeeded to the earldom on the death of his father 30 December 1794. He sat in the House of Lords as a Scottish Representative Peer between 1796 and 1806. In the latter year he was created Baron Ailsa, of Ailsa in the County of Ayr, in the Peerage of the United Kingdom, which entitled him to an automatic seat in the House of Lords.
He was admitted a Fellow of the Royal Society on 18 February 1819.
In 1831 he was created Marquess of Ailsa, of the Isle of Ailsa in the County of Ayr. He voted for the Reform Bill in 1832.
Lord Ailsa married Margaret Erskine, the second daughter of John Erskine of Dun, Forfarshire, on 1 June 1793. They had six children:
Lady Alicia Jane Kennedy (d 1887), married Jonathan Peel and had issue.
Archibald Kennedy, Earl of Cassilis (1794–1832), married Eleanor Allardyce and had issue.
Lady Anne Kennedy (1798–1877), married Sir David Baird of Newbyth, 2nd Baronet and had issue. Lady Margaret Kennedy (1800–1889), married Thomas Radclyffe-Livingstone-Eyre. Lady Mary Kennedy (1800–1886), married Richard Oswald, son of Richard Alexander Oswald, Member of Parliament for Ayr of Auchencruive Estate.
The Honorary
John Kennedy-Erskine assumed the additional name of Erskine on being named heir to the House of Dun. Lord Ailsa bought a house near Twickenham in London that had previously belonged to the playwright Richard Brinsley Sheridan. He named it "Street Margaret"s" and the name of the house has now been applied to the whole adjacent area.
Ailsa Road and Ailsa Avenue in the area are also named after him.
Royal Society.
