He was the only surviving son of Richard Fitzalan, 11th Earl of Arundel and his first wife, Mary Bohun. When he was 16 his father was executed and his lands and titles forfeited. Thomas was given as ward to the king's half-brother John Holland, 1st Duke of Exeter, along with a large part of the Arundel estates. Holland greatly mistreated him, a matter Thomas was to cruelly repay many years later.
Eventually Thomas escaped from his guardian and joined his uncle Thomas Arundel, the deposed Archbishop of Canterbury, in exile. The two eventually joined with another exile, the king's cousin Henry.
Thomas followed Henry in his return to England in July 1399, and in the following events which led to the deposition of King Richard and Henry's crowning as Henry IV. He was butler at the coronation, and shortly afterwards the new king restored him to his titles and estates. These included the Earldoms of Arundel and Surrey, and a large estate in the Welsh marches.
Early the next year a group of barons who had been close to the deposed king revolted, amongst them Thomas' former guardian John Holland. The latter was captured by followers of Thomas's aunt Joan, Countess of Hereford, and at Thomas' behest was soon executed (some claim he was tortured first).
The new few years Thomas was much occupied by events in the Welsh marches, where he had to help deal with the revolt of Owen Glendower.
In 1405 there was a revolt in the north of England, led by the Archbishop of York, Richard le Scrope, and the 2nd Duke of Norfolk, Thomas Mowbray. Thomas was the head of the commission when condemned the pair to death. This apparently led to a falling out between Thomas and his uncle Archbishop Thomas Arundel, who objected to the execution of a fellow prelate.
King Henry's sister Philippa of Lancaster had married King John I of Portugal, and to further cement the alliance between England and Portugal, Thomas married king John's illegitimate daughter Beatrice on November 26, 1405.
In the following years Thomas again had to help surpress revolts in Wales and the Welsh marches.
Politically Thomas allied himself with the king's half-brother's the Beauforts, and when Thomas Beaufort, 1st Duke of Exeter was appointed Chancellor in 1410, Arundel became one of the king's principal councilors. Beaufort favored an alliance with Burgundy, and Arundel was one of the leaders of those sent to help fight the rival Armagnac faction in France. Sometime in this period Arundel was made a Knight of the Garter.
However, in 1412 the Beauforts lost power, and Arundel retired to his estates until the next year, when Henry IV died. His son restored Arundel to a place of influence, immediately appointing him Lord Treasurer, as well as constable of Dover Castle and warden of the Cinque Ports.
Arundel was one of the intial commanders of Henry V's 1415 French campaign, but at the seige of Harfleur he, along with many others, fell ill and had to return to England. He never recovered, and died not long afterwards.
Arundel left no children. The castle and lordship of Arundel was inherited by his cousin John Fitzalan, 13th Earl of Arundel. The earldom of Surrey fell into abeyance. The rest of his property was split amongst his 3 surviving sisters.
{| border="2" align="center"
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|width="30%" align="center"|Preceded by:
Richard Fitzalan
|width="40%" align="center"|Earl of Arundel
|width="30%" align="center"|Followed by:
John Fitzalan
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