- For others similarly named, see the Robert Ainsworth navigation page
He was born at Clifton, Lancashire in September 1660. After he had finished his own education, he commenced as schoolmaster at Bolton; from there he went to London; and at Bethnal Green, Hackney, and other suburban villages, continued to keep a school, until he retired some years before his death.
Ainsworth died on 4 April 1743, at the age of 82, and was buried at St Matthias Old Church, Poplar, where an inscription in Latin verse, written by himself, was placed over his remains and those of his wife. One of the heirs of his estate was a nephew, Peter Ainsworth (born 1713), who used his uncle's money to establish a successful bleach works at Halliwell. He was the grandfather of Peter Ainsworth (1790–1870), a Whig politician.
In 1736, after about twenty years' labour, Ainsworth published his major work, with a dedication to Richard Mead, and a preface explaining his reasons for undertaking it. Improved editions by Samuel Patrick, John Ward, William Young of Gillingham, Isaac Kimber (editing 1751) and Thomas Morell successively appeared; Ward and Young's (1752) in folio, the others in quarto. Nathaniel Thomas's version was from 1758. John Carey's (1816) was a later version; there were also abridgments by Young and Morell. Another 19th-century edition was that of Benjamin Wrigglesworth Beatson with William Ellis, based on the 1752 edition. This dictionary was an improvement on all that had preceded it in England: that of Alexander Adam was a further advance.
Earlier, Ainsworth had published a treatise on education, entitled The most Natural and Easy Way of Institution (1698), in which he advocated the teaching of Latin by conversational methods and deprecates punishment of any sort. Ainsworth was author also Monumenta Vetustatis Kempiana (1720), an expansive account of the classical collection of John Kemp, of A Short Treatise on Grammar, and some smaller pieces. He is said to have been a hunter after old coins and other curiosities.
- Rose, Hugh James (1857). "Ainsworth, Robert". A New General Biographical Dictionary. London: B. Fellowes et al.
- Fuller, Tony (1998). Memorial Inscriptions at the East India Chapel, Poplar. Hornchurch: Armenians in India Press.
- Landed families of Britain and Ireland: Ainsworth of Smithills Hall and Moss, accessed 6 February 2016
- Gentleman's Magazine and Historical Review. 1837. p. 366.
- Lee, Sidney, ed. (1892). . Dictionary of National Biography. 31. London: Smith, Elder & Co.
- Stephen, Leslie, ed. (1885). . Dictionary of National Biography. 1. London: Smith, Elder & Co.
- One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Ainsworth, Robert". Encyclopædia Britannica. 1 (11th ed.). Cambridge University Press. p. 441.