As a younger brother of Edward, the Black Prince, John exercised great influence over the English throne during the minority of Edward's son, King Richard II, and the ensuing periods of political strife. Due to some generous land grants, John was one of the richest men in his era. He made an abortive attempt to enforce a claim to the Crown of Castile that came courtesy of his second wife Constance, who was an heir to the Castillian Kingdom, and for a time styled himself as King of Castile.
John of Gaunt's legitimate male heirs, the Lancasters, include English kings Henry IV, Henry V, and Henry VI. His other legitimate descendants include his daughters Queen Philippa of Portugal and Elizabeth, Duchess of Exeter (by his first wife Blanche of Lancaster), and Queen Catherine of Castile (by his second wife Constance of Castile). John fathered five children outside marriage, one early in life by a lady-in-waiting to his mother, and four by Katherine Swynford, Gaunt's long-term mistress and third wife. The children of Katherine Swynford, surnamed "Beaufort," were legitimised by royal and papal decrees after John and Katherine married in 1396. Descendants of this marriage include Joan Beaufort, Countess of Westmorland, a grandmother of kings Edward IV and Richard III; John Beaufort, 1st Earl of Somerset, a great-grandfather of King Henry VII; and Joan Beaufort, Queen of Scots, from whom are descended all subsequent sovereigns of Scotland beginning in 1437 and all sovereigns of England, Great Britain and the United Kingdom from 1603 to the present day. The three houses of English sovereigns that succeeded the rule of Richard II in 1399 — the Houses of Lancaster, York and Tudor — were all descended from John's children Henry IV, Joan Beaufort and John Beaufort, respectively. In addition, John's daughter Catherine of Lancaster was married to King Henry III of Castile, which made him the grandfather of King John II of Castile and the ancestor of all subsequent monarchs of the Crown of Castile and united Spain. Through his daughter Philippa, he was grandfather of King Edward of Portugal and an ancestor of all subsequent Portuguese monarchs as well. Through John II of Castile's granddaughter Joanna the Mad, John of Gaunt is also an ancestor of the Habsburg rulers who would reign in Spain and much of central Europe.
John of Gaunt's eldest son and heir, Henry Bolingbroke, Duke of Hereford, the son of his first wife Blanche of Lancaster, was exiled for ten years by King Richard II in 1398 as resolution to a dispute between Henry and Thomas de Mowbray, Duke of Norfolk. When John of Gaunt died in 1399, his estates and titles were declared forfeit to the crown, since King Richard II had named Henry a traitor and changed his sentence to exile for life. Henry Bolingbroke returned from exile to reclaim his inheritance and depose Richard. Bolingbroke then reigned as King Henry IV of England (1399–1413), the first of the descendants of John of Gaunt to hold the throne of England.
John was the fourth son of King Edward III of England. His first wife, Blanche of Lancaster, was also his third cousin; both were great-great-grandchildren of King Henry III. They married in 1359 at Reading Abbey as a part of the efforts of Edward III to arrange matches for his sons with wealthy heiresses. Upon the death of his father-in-law, the 1st Duke of Lancaster, in 1361, John received half his lands, the title "Earl of Lancaster", and distinction as the greatest landowner in the north of England as heir of the Palatinate of Lancaster. He also became the 14th Baron of Halton and 11th Lord of Bowland. John inherited the rest of the Lancaster property when Blanche's sister Maud, Countess of Leicester (married to William V, Count of Hainaut), died without issue on 10 April 1362.
John received the title "Duke of Lancaster" from his father on 13 November 1362. By then well established, he owned at least thirty castles and estates across England and France and maintained a household comparable in scale and organisation to that of a monarch. He owned land in almost every county in England, a patrimony that produced a net income of between £8,000 and £10,000 a year.