Guillaume was the second son of Jannequin de St Martin, Seigneur of Craqueville, and Collette de La Rocque.
A fierce feud existed between Guillaume and his brother Jannequin, Raulet, James, and Guy, on one side and their cousin, Thomas de St Martin, Seigneur of Trinity. In 1447 Thomas obtained Letters of Protection from the King "for fear of Jannequin and Raulet de St Martin", who were threatening him, his provost, and his tenants.
In January 1452 Thomas, who with his son Thomas, had been taken prisoner by the French, in order to pay their ransom, sold Trinity Manor to his brother-in-law, Thomas de La Court of Guernsey, who had married his sister, Jenette. The family strenuously resisted the sale and first his brother, John, and then Guillaume, his cousin, tried to annul it by claiming the retrait lignager. These family dissensions reached such a pitch that in February 1452 Thomas de La Court had to obtain Letters of Protection from the Earl of Warwick, Lord of the Isles.
Within a year, however, Thomas de St Martin went over to the French allegiance. This, by an ordinance of Henry VI, made the sale null and void but Thomas de La Court, the son of the purchaser, obtained from the King a Letter Patent ratifying it, and on 16 September 1456 the Jersey Royal Court adjudged him to be rightful possessor of the Fief. Meanwhile, from October 1452 to January 1459, Guillaume and his brother James had held some official position in Jersey, for in the latter month a Commission was appointed to audit their accounts.
When the Wars of the Roses broke out in England, the dispute was embittered, for the de St Martins became Lancastrians and the de La Courts Yorkists. The position was further complicated by a quarrel between the de St Martins and Otis Colin, the Lieutenant of Mont Orgueil.
In January 1457 Jannequin appeared before the Royal Court, and showed wounds caused by soldiers from the Castle, and asked to be placed under the King's Protection for fear of the Lieutenant and his followers. The Court ordered the Viscount to warn him not to interfere with him or arrest him without the Court's order. A month later the brothers' mother, now a widow, complained to the Court that Colin was holding three her of her sons, Guillaume, Raulet, and James, prisoners in the Castle, and asked that they might be presented for trial to answer the charges against them. Colin then removed them to Guernsey, out of the jurisdiction of the Jersey Court, and they remained for 18 months in a dungeon in Castle Cornet.
In September 1458 their mother appeared before the Guernsey Court pleading that they were suffering grievous bodily injury for lack of food, and were detained in contempt of the King's Letter Patent. The Court ordered them to be sent back to Jersey for trial. Here our information ends; but the order was apparently obeyed, for in 1460, after the defeat of the Yorkists at Ludford the flight of Warwick to Calais and the forfeiture of all his estates including "our island of Jersey with all the other islands". On 5 June Guillaume de St Martin was made Attorney-General of Jersey and his brother Raulet Comptroller-General of Jersey and Guernsey.
In 1461 Mont Orgueil was `surprised' by troops of Pierre De Breze, Seneschal of Normandy. He was first cousin of Marguerite of Anjou, Henry VI's Queen, the moving spirit of the Lancastrian cause; and this was possibly done with the connivance of Nanfan, the Lancastrian Warden. In this transaction Guillaume de St Martin played a notorious but not clearly definable part. The Extente of 1528 records: "Forfeiture of St Martins, that is to say Guille, Raulet, Jamys, Guy, John, and Thomas de St Martin, by reason of treason of selling the King's Castle in Jersey".
In 1463 Renaud Lempriere, Seigneur of Rosel, was accused of saying:
- "Cursed be the man who brought the French into the island, that false traitor, Guillaume de St Martin, who sold us like meat on a butcher's stall".
De Breze continued Guillaume in office as Attorney-General, and in December 1463 he was one of the tribunal of three who tried Renaud Lempriere and Thomas Le Hardy for conspiring to expel the French. Other members of the family served in the French garrison.
He also took possession of the disputed Manor of Trinity. Warwick the Kingmaker, as Lord of the Isles, declared all the de St Martin estates forfeited, and gave them to the Guernsey cousin, Thomas de La Court; but, as Jersey remained for seven years under Norman rule, the latter could not obtain them until the French were expelled. When the English recaptured the Castle in 1468, the brothers fled to Normandy, where the French King gave them the Manor of Breuil d’Anneville, which had been granted to their father by Henry V, when the English conquered Normandy. Here one of their descendants was still living 30 years later.