Since Thomas was still in his minority at the time of his father's death: Domhall, King
Edward III of England, with whom
Scotland was battling in the
Scottish Wars of Independence, placed Thomas in the care of his (Thomas's) step-father, William Carsewell. He was prevented from receiving the earldom by
Richard Talbot, 2nd Baron Talbot, who claimed the title
Lord of Mar in the right of his wife,
Elizabeth de Comyn. In the tumultuous years that followed Dupplin Moor, Thomas's grandmother, Lady
Christina Bruce, held the seat of the earls of Mar,
Kildrummy Castle. When she died in 1357, the castle passed to Thomas along with her lands and lordship, which were called the Earldom of
Garioch. In 1351, Thomas was one of the
ambassadors sent to
England to negotiate the
ransom of Scotland's king,
David II, who was prisoner there. When David was eventually released in 1357, Thomas was one of the seven lords “from whom three were to be selected as hostages” until the king's ransom was paid. As Earl of Mar (as he was styled in 1357), Thomas was made
Great Chamberlain of Scotland in 1358. Thomas is said to have favored England's king, Edward III, inasmuch as the English king had granted him a pension of 600
merks per annum. In addition, the English king agreed to pay Thomas £600 sterling yearly if he lost his lands in Scotland. Thomas agreed to serve the English king by fighting in England's war with
France in 1360. In 1362, he was sent as a Scottish ambassador to negotiate with England, and in 1369 he was one of the guarantees of a truce between the two nations. Earl Thomas was in both England and France frequently in his life, as John Mackintosh has laid out in his book
Historic Earls and Earldoms of Scotland: In March, 1359, he had a passport through England for himself and thirty persons in his retinue, and three merchants; while in August, 1359, he had a
safe conduct for himself and one hundred horsemen in his train. The same year, in October, he had a passport to France with twenty-four horsemen. In November, 1362, he had a safe conduct to the
shrine of
St. Thomas a Becket at
Canterbury for himself and twelve horsemen. He had passports for himself and twelve horsemen in February, 1363, in March the same year, and in February, 1365. In July, 1365, he had a licence to send eight horsemen to
Newcastle-on-Tyne with one hundred and twenty oxen, which he had sold to merchants in that city. In October, 1368, he had a passport for himself and twelve
gentlemen on their way through England in
pilgrimage to St. [John of]
Amiens, in France. In 1363, he fell out of favor with David II, perhaps due to David's resentment of Thomas's alliance with the English or perhaps because of the extortions Thomas was alleged to have committed on his people. For whatever reason, David besieged and took his Castle of Kildrummy. However, in 1368, upon payment of a
composition, Thomas received it back and was restored to the king's good favor. ==Marriages==