He was born in Gent, in Flanders, from which his name, John of Gaunt, derives. Aged nineteen, he married his cousin Blanche, heiress of Lancester. When her father died in 1362, he became Duke of
Lancaster and the greatest landholder in England.

John joined his brother, the Prince of Wales, on his Spanish campaign to reinstate Pedro III 'the Cruel', King of Castile, who had been deposed by his half-brother Enrique of Trastamara. After Blanche
died in 1369, John of Gaunt married Constance, daughter of Pedro 'the Cruel' and, in 1372, claimed the Castilian crown in his wife's right, as her father had been murdered in 1369 by Enrique of Trastamara.
In the early 1370s he campaigned against the French around La Rochelle and Bordeaux and returned to England in 1375. Where he supported the court faction led by Alice Perrers, his father's mistress. However, widespread opposition to Alice led to the Good Parliament of 1376, which ousted her and her followers, but John was able to reverse most of its decrees in 1377.

In an effort to undermine his clerical opponents, he supported John Wycliffe's anti-clerical theology and defended Wycliffe during his trial in 1377. During the Peasants Revolt of 1381, the rebels sacked and burned the Savoy palace, his London home. In 1386 John of Gaunt left England to try to win the Castilian throne but was unsuccessful. In 1388 he relinquished his claims to his daughter, Catherine, who was then married to the future Enrique III of Castile.

On his return to England in 1389, he acted as peacemaker between his nephew, King Richard II, and the lords apellant. However, his appointment as Duke of Aquitaine in 1390 revived the baron's
hostility. In 1394 Constance, his second wife, died and in 1396 he married his longstanding mistress, Catherine Swynford. In his last years his relationship with the king became increasingly strained and
in 1398 Richard II exiled John's son Henry Bolingbroke (later Henry IV).
 

Source: Leo van de Pas