Ainsworth, Sir John Francis (1912–81), 3rd baronet, palaeographer, and archivist, was born 4 January 1912 in London, the only son of Thomas Ainsworth, later 2nd baronet, a racehorse breeder and master of various packs of foxhounds in Ireland, and his first wife, Edina Dorothy Hope (d. 1964), fourth daughter of the 4th Marquess Conyngham of Slane, Co. Meath, where in his youth John spent much time. He distinguished himself at Eton and became a senior scholar at Trinity College, Cambridge, where he graduated BA in classics (1933). From 1937 to 1940 he was honorary editor of the British Record Society. His extraordinary palaeographical skills induced Edward MacLysaght (qv) to recommend his appointment in 1943 as inspector of manuscripts with the Irish Manuscripts Commission (later with the NLI); he held the post until his death, though it was never made permanent, nor was it very remunerative. He wrote a long series of reports on manuscripts in private keeping and edited various documents, most notably The Inchiquin manuscripts (IMC, 1961). He was also a tutor in the archives department of University College, Dublin, from its inauguration (1969). Ainsworth was modest about his scholarship, regarding himself as an archivist, not a historian.
From 1964 until his death ‘Johnny’ Ainsworth was vice-chairman or chairman of the Dublin Society for Prevention of Cruelty to Animals. He was also first whipper-in of the Curragh beagles. His first marriage (1938), to Josephine Bernard, was dissolved in 1946, in which year he married Anita Margaret Ann Lett of Kilgibbon, Enniscorthy, Co. Wexford; both marriages were childless. He lived at Carraphouca, Shankill, Co. Dublin. Ainsworth succeeded to the baronetcy on 1 March 1971. He died 30 April 1981 and was buried at Slane; he was succeeded as baronet by his half-brother Thomas (1926–2000), a horse breeder of Crotanstown, Co. Kildare.