Thomas was born of royalty in 1388. He was not allowed to inherit any lands due to his father's treason. However, Thomas did recover his reputation and wealth during his life.

Thomas was fourth Earl of Salisbury, knight of the Garter in 1414, joint commissioner to arrange marriage of Henry V and French Princess Catherine, Count of Perche in about 1419, Lieutenant-general of Normandy in 1419, governor of Champagne and Brie in 1422, and lieutenant-general of the field in 1428. Thomas was described as being imaginative, daring, skillful, having relentless drive and the personality to inspire confidence.

Thomas was mostly known as a skillful commander on the English side of the Hundred Year War. Before the Hundred Year War the national character of England and France were somewhat unformed. The Hundred Year War was the last war of the heavy knight in armor. He was said to be in his time, through France and England, the most expert, subtle, and successful-in-arms of all the commanders who had been talked about in the last two hundred years. He had all the virtues of a true Knight, for he was gentle and humble and courteous. He was liberal with all he possessed. He gave alms freely. To the lowly he was kind and full of sympathy. To haughty enemies he was like a lion or tiger.

Henry V: The Scourge of God says, "… from the very beginning of his reign Henry displayed extraordinary self-confidence... The Earls of Huntingdon, Oxford, and Salisbury - sons of the conspirators of 1400 - had their family estates restored..." It continues, "he wars in France turned the higher nobility into professional soldiers,' says G. L. Harriss. Foremost among these soldier noblemen were the Earls of Salisbury, Warwick, and Huntingdon... Thomas Montacute, Earl of Salisbury, a year younger than the king and the son of Richard II's favourite, was the most brilliant commander of the entire Hundred Years War after Henry himself. Henry had total trust in him - although to begin with he may have had reservations because of his parentage. A complete professional, he was a daring raider into enemy territory who could extricate his men from the most dangerous situations; at the same time he was a skilled artilleryman and expert in siegecraft, like the king, and no less sound on staffwork or in finding supplies. Above all, he had a shrewd grasp of strategy and tactics. Although a ferocious disciplinarian he was popular with the troops. He was dreaded by the enemy. Shakespeare probably conveys accurately enough what the French thought about him:

Salisbury is a desperate homicide, He fighteth as one weary of his life.

His ways with prisoners did not endear him to the French - after capturing the chateau of Orsay in 1423 he brought the garrison back to Paris with ropes round their necks."

As an example of Thomas's resourcefulness and ruthlessness at times is the following. At Orleans Thomas brought up the archers, extricated what remained of the English force with considerable difficulty. To cross the Loire he had to construct a bridge out of carts and fencing. He found an even more ingenious way of bridging the River Sarthe. He made his men wear white crosses like dauphinists and, having convinced some local peasants that he was a Frenchman, ordered them to build a bridge for him. Once over he had the peasants put to death.

Henry V at siege of Harfleur and battle of Agincourt (1415) (Agincourt is one of the most famous battles in English and military history, this Hundred Years War battle is one of the battles popularly credited with ending the era of the knight); the component he contributed/commanded to Henry's army was one of the largest. At Agincourt, Thomas provided and commanded 40 men-at-arms and 80 horsed archers. In comparison, the King's uncle, the Duke of York, provided 100 men-at-arms and 300 horsed archers. Thomas's force was typical of those provided by the richest English nobility (much of which was at Agincourt). Thomas's command was roughly 8th largest among the English forces.

Holinshed's Chronicle says of Thomas, "This earl was the man at that time, by whose wit, strength, and policy, the English name was much fearfull and terrible to the French nation, which of himself might both appoint, command, and do all things in manner at his pleasure, in whose power (it appeared after his death) a great part of the conquest consisted: for surely, he was a man both painfully diligent, and ready to withstand all dangerous chances that were at hand, prompt in council, and of courage invincible, so that in no one man, men put more trust; nor any singular person won the hearts so much of all men."

Thomas entered Paris with Henry in 1420. Thomas had possesion of Jargeau, Meung, Beaugency, and Janville before he laid siege to Orleans.

Thomas was killed in an artillery accident on November 1, 1428, eight days after the accident that happen late Sunday afternoon October 24, 1428, while besieging Orleans. In Holinshed's Chronicle the following account is given of Thomas's death. "In that tower that was taken at the bridge end (as before you have heard) there was a high chamber, having a grate full of bars of iron, by which a man might look all the length of the bridge into the city; at which grate many of the chief captains stood many times, viewing the city, and devising in what place it was best to give the assault. They within the city well perceived this looking hole, and laid a piece of ordinance directly against the window. It so chanced, that the nine and fiftith day after the siege was laid, the earl of Salisburie... with diverse other went into the said tower, and so into the high chamber, and looked out at the grate, and within a short space, the son of the master-gunner, perceiving men looking out at the window, took his match (as his father had taught him) who was gone down to dinner, and fired the gun; the shot whereof broke, and shattered the iron bars of the grate, so that one of the same bars struck the earl so violently on the head, that it struck away one of his eyes, and the side of his cheek. Sir Thomas Gargraue was likewise striken, and died within two days. The earl was conveyed to Meung on Loire, where after eight days he likewise departed this world, whose body was conveyed into England with all funeral appointment, and buried at Bissam by his progenitors, leaving behind him an only daughter named Alice, married to Richard Nevill." Taken to Meung after his grisly injury during the siege, it was here he died.

After Thomas's death, the gook luck, which had followed the English nation, began to decline. It was at this time that Joan of Arc rose to power and it was said that Thomas's death was an act of God and that Joan would have crushed him if he were still fighting.

References:
Holinshed's Chronicle - 1577
Henry V: The Scourge of God
Montaguemillennium.com