FitzGerald, John fitz Thomas (c.1198–1261), magnate, was the son and heir of Thomas fitz Maurice FitzGerald (d. 1213), lord of Shanid, and his wife, Sadhbh. On his father's death, custody of his wardship and marriage was granted to Thomas fitz Anthony (qv), seneschal of Leinster and the de facto lord of Decies and Desmond, who married fitz Thomas to his daughter Margery and endowed him with lands in north Kerry. After fitz Anthony's death, Decies and Desmond escheated to the crown, though in practice it remained divided between fitz Anthony's four daughters and their husbands. In 1234 fitz Thomas sided with the king against Richard Marshal (qv), though there is no evidence to suggest that he was involved in Marshal's murder after the battle at the Curragh. This proved to be the turning point in his career: his brothers-in-law had not been so discerning and were refused royal permission to enter their purparty of the lordship, but John received the king's effusive thanks. In 1242 he accompanied Henry III on his campaign to Poitou and in 1244 was summoned to the abortive Scottish expedition. He was briefly appointed sheriff of Limerick in the 1250s and between 1250 and 1259 crossed over to England on at least six occasions to lobby the king and the Lord Edward personally for a grant of his father-in-law's entire lordship. Finally, on 7 November 1259, Edward gave him Decies and Desmond, together with the castle of Dungarvan to hold in fee for 500 marks. Ignoring the protestations of the justiciar, Stephen Longespée (qv), who refused to give him seisin, claiming that Edward had been deceived, fitz Thomas forcefully took possession of the lordship early in 1260.
Much of his career was spent campaigning against the MacCarthys in south-west Munster. In 1234 he rushed to defend Tralee from a raid by Diarmait son of Cormac Óc, and in 1244 he killed Domnall Ruad. In 1250 he slew Domnall Cairprech, an ally of the Cogans, and the following year, completely disregarding royal letters of protection, he murdered Domnall Got (qv). Between 1257 and 1260 he mounted a number of expeditions into Thomond and in recognition of his successes against the O'Briens he was granted lands there. In 1260, in order to combat the growing threat of Fíngen MacCarthy (qv), the son of Domnall Got, he sought aid from the new justiciar, William de Dene. In return for an undertaking by fitz Thomas and the other magnates in Munster to finance the campaign, the justiciar agreed to mount an expedition. On 24 July 1261 the entire army led by fitz Thomas and Dene was annihilated by Fíngen at Callann near Kenmare. Fitz Thomas, his eldest son Maurice fitz John, eight Munster barons, and some twenty-five knights were killed. John and his son were buried at the Dominican friary at Tralee, which he had founded in 1243. Despite Fíngen MacCarthy's death shortly after Callann, much of the lordship of Desmond went to waste during the long minority of fitz Thomas's grandson and heir, Thomas fitz Maurice FitzGerald (qv) (d. 1298).
John's marriage to Margery produced Maurice. In addition he had four other sons: Sir John of Glincarbery, ancestor of the knights of Glin, Gilbert of Meane, ancestor of the white knights, Philip, and Maurice, ancestor of the knights of Kerry.