FitzGerald, John fitz Thomas (c.1265–1316), 5th lord of Offaly and 1st earl of Kildare , magnate, was son of Thomas fitz Maurice FitzGerald (d. 1271), the fourth son of Maurice fitz Gerald FitzGerald (qv) (d. 1257), 2nd lord of Offaly. He emerged from obscure beginnings in Connacht in June 1287 to become one of the foremost magnates of the country by his death. His rise to prominence began when his cousin Gerald fitz Maurice FitzGerald (qv) (d. 1287), 4th lord of Offaly, began to transfer his lands into fitz Thomas's hands. Fitz Maurice's decision to give his lands to his male heir rather than letting them pass to the de Cogan family through his aunt, Juliana, who had married John de Cogan II, also by-passed the daughters of Maurice fitz Maurice FitzGerald (qv) (d. 1286), uncle of both fitz Maurice and fitz Thomas. Fitz Thomas also benefited when Amabilia, one of fitz Maurice's daughters, gave him all her lands in Connacht (February 1288). As a result of these grants fitz Thomas gained most of the lands of the Geraldines of Offaly, with large estates in Connacht and Leinster and claims to lands in western Ulster. However, a large portion of the lands of the third lord of Offaly, Maurice fitz Gerald FitzGerald (qv) (d. 1268), remained in the hands of his widow Agnes de Valence (qv), a cousin of Edward I, and these became a focus of controversy between Agnes and fitz Thomas. He developed close ties with John of Sandford (qv), archbishop of Dublin, serving with him in Offaly (September 1289) and joining his entourage when Sandford went to England (1291). While in England he seems to have gained easy access to the king, who granted him custody of the royal castles of Roscommon and Rinndown. Fitz Thomas's ambitions in Connacht brought him into conflict with two of the most important lords of the day, William de Vesci (qv), lord of Kildare and justiciar, and Richard de Burgh (qv), 2nd earl of Ulster; and these conflicts gave rise to his modern reputation of extreme lawlessness. His quarrel with de Vesci started in 1293 when he deposed de Vesci's choice for the O'Connor kingship in Connacht. De Vesci retaliated using his powers as lord of Kildare, and by July 1293 the king had to intervene to prevent open warfare. Fitz Thomas was challenged to a duel by de Vesci after he openly slandered the justiciar in April 1294, but failed to appear, and eventually the case against him was dropped (1295). De Vesci never returned to Ireland, leaving fitz Thomas to dominate the region around Kildare.
Fitz Thomas's quarrel with the earl of Ulster was less successful. He instigated a ‘time of disturbances’ by attacking the earl's men throughout Ireland (1294) and instigating the destruction of the records of the liberty of Kildare; he then captured de Burgh (December 1294) and imprisoned him for three months till he relinquished all authority over fitz Thomas in Connacht. He was summoned before the king (July 1295) and allowed to return home only after agreeing to strict conditions of behaviour. A truce was arranged in 1296 by the justiciar, John Wogan (qv), and a permanent agreement was made (October 1298) in which fitz Thomas surrendered all his lands in Connacht to de Burgh; but he did so slowly, and thus forfeited lands in Munster and Leinster offered in partial compensation. Fitz Thomas also pursued extreme tactics against Agnes de Valence in pursuit of the Geraldine lands in Limerick, suborning local officials, refusing to honour debts, and twice seizing the lands on reports of her death.
Set against this lawless behaviour was his frequent overseas service to the king in Scotland (1296, 1301) and Flanders (1298), which brought him back into royal favour. In Ireland he campaigned regularly against the Gaelic lords but was never entrusted with the office of justiciar. In 1298 he was granted the lands of his distant cousin Thomas fitz Maurice FitzGerald (qv) (d. 1298) of Desmond during the minority of the heir, and seems to have settled into a more law-abiding period. However, after the death of Agnes de Valence (1310) he almost resorted to open warfare against John de Cogan III over the Geraldine lands in Limerick; but de Cogan died soon afterward, leaving fitz Thomas in possession of the lands. By this time he was recognised as one of the foremost magnates in Ireland, and his children by his wife, Blanche de Roche (m. a.1285?), made good marriages, most notably his son Thomas to Joan, daughter of the earl of Ulster, and his daughter Joan to Edmund Butler (qv). His loyalty and service to the crown during the Bruce invasion brought him his final honour. He was summoned to England to discuss the situation and was created earl of Kildare in May 1316 and granted all the lands and rights formerly held by de Vesci in Kildare, with the important exception of the liberty of Kildare. However, he did not live long to enjoy his new status, as he died in September 1316 and was buried at the Franciscan friary at Kildare, leaving his new earldom to his son Thomas (qv).