Thomas Cantock was an English-born cleric and judge in medieval Ireland, who held the offices of Bishop of Emly and Lord Chancellor of Ireland. O'Flanagan, writing in 1870, complained that the biographer of Cantock was faced with an almost complete lack of information about his life, and not a great deal more is known about him today. He was English by birth, and was probably a relative of the Roger de Cantock who held the manor of Dyrham, Gloucestershire in 1347, and died in 1349; the family had earlier connections with Bristol. Roger's father, also named Roger, visited Ireland during Thomas's tenure as Lord Chancellor, possibly to solicit a favour from him. Thomas began his career as a clerk in royal service; he was sent by King Edward Ion a mission to Scotland in 1288 with the title of Master. He was known to be a man "learned in the law", and no doubt this was the reason why he was appointed Lord Chancellor of Ireland. He served from 1292–1294, and he held the office again from 1306–1308. He was Canon of Emly and prebendary of Mollagymon, Cashel, County Tipperary. He was elected Bishop of Emly in 1306, and installed as bishop in a ceremony at Christchurch Cathedral, Dublin, which was followed by a banquet at which he feasted first the nobility, and then the citizenry of Dublin with a magnificence "unheard of in those times". In 1307 he was granted a third part of the manor of Ballyfermot by William Fitzwilliam and his wife Avice, who were members of the family which in time would come to own most of Dublin south of the River Liffey. He is also known to have acquired lands at Chapelizod. He died on 4 February1309. O'Flanagan states that from the little that is known of him he seems to have been popular and hospitable. According to Smyth, during his tenure as Lord Chancellor a serious fire destroyed all the Irish Chanceryrecords; this is almost certainly a reference to the Great Fire of Dublin of 1304, which destroyed much of the medieval city.
