Thomas Tonkin (1678–1742) was a Cornish landowner and historian.
He was born at Trevaunance, St Agnes, Cornwall, and baptised in its parish church on 26 September 1678, was the eldest son of Hugh Tonkin (1652–1711), vice-warden of the Stannaries 1701, and High Sheriff of Cornwall 1702, by his first wife, Frances (1662–1691), daughter of Walter Vincent of Trelevan, near Tregony.
Tonkin matriculated from Queen's College, Oxford, on 12 March 1693–4, and was entered as a student at Lincoln's Inn on 20 February 1694–5. At Oxford he associated with his fellow-collegian, Edmund Gibson, afterwards bishop of London, and with Edward Lhuyd, who between 1700 and 1708 addressed several letters to him in Cornwall and he was friendly with Bishop Thomas Tanner.
Tonkin withdrew into Cornwall and settled on the family estate. From about 1700 to the end of his days he prosecuted without cessation his inquiries into the topography and genealogy of Cornwall, and he soon made 'great proficiency in studying the Welsh and Cornish languages'; but he quickly became involved in pecuniary trouble. To improve his property he obtained in 1706 the queen's sign-manual to a patent for a weekly market and two fairs at St. Agnes, but through the opposition of the inhabitants of Truro the grant was revoked. His progenitors had spent large sums from 1632 onwards in endeavouring to erect a quay at Trevaunance-porth. By 1710 he had expended £6,000. upon it, but the estate afterwards fell 'into the hands of a merciless creditor,' and in 1730 the pier was totally destroyed 'for want of a very small timely repair and looking after'.
Tonkin's wife was Elizabeth, daughter of James Kempe of the Barn, near Penryn. Thomas Worth, jun., of that town, and Samuel Kempe of Carclew, an adjoining mansion, were his brothers-in-law. He had by these connections much interest in the district, and from 12 April 1714 at a by-election, to the dissolution on 5 January 1714–15, he represented in parliament the borough of Helston. Alexander Pendarves, whose widow afterwards became Mrs. Delany, was his colleague in parliament and his chief friend; they were 'Cornish squires of high tory repute'.
On the death of the last of the Vincents, Tonkin dwelt at Trelevan for a time; but the property was too much encumbered for him to retain the freehold. The latter part of his life was passed at Polgorran, in Gorran parish, another of his estates. He died there, and was buried at Gorran on 4 January 1741–2. His wife predeceased him on 24 June 1739. They had several children, but the male line became extinct on the death of Thomas Tonkin, their third son.