George Balleine's A Biographical Dictionary of Jersey says that he was the eldest son of Jean de St Martin, who was buried at St Saviour's in 1462, and Jenette Le Hardy. However, it seems more likely that it was another Thomas who married Jenette and that they were the parents of the Thomas with whom we are concerned here.
When the Norman troops of Pierre De Breze surprised Mont Orgueil in 1461, Thomas was involved with his cousins in the rather mysterious treason of "selling and delivering the King's Castell". On the expulsion of the French in 1468 he fled to Normandy and his estates were confiscated. On 16 October 1480, however, he received a Pardon from Edward IV, "that he may freely enter into certain lands, lordships, and fees of his ancestors, to whom he is heir, in the islands of Gersey and Guernsey, as of the fee called the Fee de la Trinite".
Then followed a lawsuit with Thomas de La Court, whose grandfather had bought Trinity Manor in 1432. De St Martin claimed that this sale had been invalidated by a challenge under the Retraii lignager but the Court decided against him. De St Martin then petitioned the King. In 1485, when Thomas Hutton arrived as Commissioner to inquire into the administration of the Channel Islands, an agreement was made in his presence, by which de La Court gave up the Manor for payment of 140 crowns.
Meanwhile Henry VII by the Battle of Bosworth (22 August 1485) had seized the throne. On 28 November, within a month of coronation, a grant was made to Edmond Weston and Thomas de St Martin, "in consideration of good and gratuitous services performed by them with great labour and great personal cost to themselves", of the office of Captain, Keeper, and Governor of the island of Guernsey "to hold the same in survivorship without rendering any accompt thereof".
What those services were is not known, but they must have been connected with the expedition that gave Henry the crown. De St Martin went to Guernsey, for the colectors of customs at Poole received £8 for their expenses in sending him there; but four months later he dropped out of the Governorship, and Edmond was made sole Governor life. De St Martin then received a post at Court.
On 31 August 1486 he is mentioned as "one of the Gentlemen Ushers of King's Chamber", for which he received an annuity of £12. On 1 June 1488 he is described as "Gentleman Usher with our dearest son, the Prince" (ie Prince Arthur), and received £5 "for the costs in going upon our message into our Isle of Guernsey". In 1493 and 1497 we find him styled Premier Usher to the Prince of Wales.
But he did not lose touch with Jersey. On 24 May 1499 he appeared before the Royal Court in St Helier, acting for the executors of 'that venerable and discreet person Jean Neel, deceased, for the accomplishment of his promise to found and endow schools'.
His will was drawn up in Jersey In 1514 (a copy is in La Société Jersiaise Library), and signed in the presence of Richard Mabon, the Dean. He left a silver chalice to Trinity Church, and money for masses to be said in that church for his own soul, and those of his wife, brother, sister and his friend Jean Neel, and for bread to be distributed to the poor who assisted at these services.
He had married Anne Brocas, and died at Easter 1515, leaving no children. He was succeeded by his nephew Drouet Lempriere, son of his sister Thomasse.