St Thomas is situated in the Deanery of Trigg Major and the Hundred of East. It is bounded on the North by St Stephens, on the East by St Stephens and Launceston, on the South by South Petherwin, and on the West by Trewen and Egloskerry. In the 1841 and 1851 Censuses, the parish was split between the borough of Launceston and a rural part which, for some reason had always been known as St. Thomas Hamlet or St. Thomas Street (or both); however, it is a part of St Thomas-by-Launceston parish. This became an enumeration district in these early censuses, and it is in these that its inhabitants recorded their origins. St Thomas-by-Launceston is now a parish wholly within the borough of Launceston. There is only one village in this parish and that is Tregadillet