He had two older brothers who lived to adulthood, and two younger brothers who did likewise. As the third of five sons, he was was incredibly unlikely to come to the throne or father a king, especially since both his elder brothers had heirs. As the Duke of Lancaster, he seems to have been rich and powerful enough to suit himself, because he never tried to overthrow his nephew King Richard II when the monarch was still a child. Furthermore, when John of Gaunt died on 3 February 1399, King Richard II sat on the reasonably secure upon the throne and Gaunt’s eldest son and heir (Henry Bolingbroke) was exiled to France, so the idea that he had sired multiple kings would have been too far-fetched for Gaunt to have contemplated. Nevertheless, fate is a tricky thing …
One thing in John of Gaunt’s favor was that he had a LOT of offspring by his three wives. (To learn more about these women, I recommend Amy License’s book Red Roses.) His first wife was Blanche of Lancaster, who gave birth to several children, three of whom lived to adulthood: his daughters Queen Philippa of Portugal and Elizabeth, Duchess of Exeter, and his son the aforementioned Henry Bolingbroke. After Blanche’s death, Gaunt married the Infanta Constance of Castile in 1371, briefly declaring himself King of Castile and Leon in her name. His second wife had two children, but only their daughter lived. However, that daughter grew up to become Queen Catherine of Castile.
Although Gaunt was nice enough to his wife Constance, his true love was a widow by the name of Katherine Swynford. He fathered more children with Katherine during his 2nd marriage than with his wife, and after Constance died, Gaunt married the Widow Swynford in 1396. Even better, he was able to get their four adult children declared legitimate and given the surname Beaufort in 1397, so that John Beaufort, 1st Earl of Somerset, Henry Beaufort (Bishop of Winchester and Cardinal), Thomas Beaufort, Duke of Exeter, and Joan Beaufort all became his legal heirs as well.
No one really foresaw the first king to come from Gaunt’s line, Henry IV. It all came about because King Richard II hated Henry Bolingbroke. Henry had joined other peers in defying the crown, and King Richard never forgave him for it. Thus, as soon as John of Gaunt died, King Richard declared all his lands and titles forfeit to the crown and forbade Bolingbroke from returning to England. Bolingbroke, extremely vexed, came back anyway, gathered forces, and stole Richard’s throne. John of Gaunt’s son was crowned King Henry IV, whose linage would include Kings Henry V, and Henry VI.
It should be noted that King Henry IV was a bit nervous about someone doing to him what he had done to Richard, so when he became monarch, he declared that his legitimate half-siblings, the Beauforts, would henceforth be unable to ever inherit the throne of England, should it come to that. The Beauforts seemed fine with this stipulation, because what were the odds of it ever being an issue?
That’s when fate started giggling.
Henry IV’s grandson, King Henry VI, was intermittently mentally ill, so his cousin Richard, 3rd Duke of York, who was the grandson of Edmund of Langley, 1st Duke of York — the fourth surviving son of King Edward III — started a civil war for the crown. This was the beginning of the bloody nightmare known as the War of the Roses. Although York died in battle, his sons eventually won and his eldest son became King Edward IV. The newest monarch was also a descendant of John of Gaunt’s via his daughter Joan Beaufort, Countess of Westmorland, who was his maternal grandmother. This means, of course, that Edward’s brother, who was eventually crowned King Richard III, was likewise a descendant of Joan Beaufort.
But surely two kings coming from the Beauforts was a fluke? After all, it was through their father’s decent from the 1st Duke of York that they made their claim to the throne. The fact that their mother was Joan Beaufort’s daughter was just the result of the cousin intermarriage so common in that time.
Well, shortly after Richard III became soverign, the plot thickened to the point it was viscous. Gaunt’s eldest son by Swynford, John Beaufort, had married Margaret Holland and fathered six kids who lived to adulthood. Their eldest son, John Beaufort, 1st Duke of Somerset, had one child — Lady Margaret Beaufort.
Her second husband was Edmund Tudor, 1st Earl of Richmond, the son of Dowager Queen Catherine of Valois by a cousin of the last true Prince of Wales, Owen Tudor. Margaret and Edmund’s only child, a little boy named Henry. The adult Henry now gathered troops and challenged Richard III for the kingdom.
The Welsh upstart defeated King Richard III on Bosworth Field and became King Henry VII. The current monarch, Queen Elizabeth II, is a descendant of Henry VII’s daughter Margaret Tudor, and thus a direct descendant of John of Gaunt. Moreover, since one of John of Gaunt’s granddaughters through John Beaufort was 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 modern queen is a product of the Beaufort line coming and going!
Additionally, through his eldest daughter Philippa, who married King John I of Portugal, Gaunt was the maternal grandfather of King Edward of Portugal … and thus “an ancestor of all subsequent Portuguese monarchs as well.”
Ergo, John of Gaunt is the progenitor of the royal families of England, Scotland, and Portugal.
But it isn’t all of it. Since John of Gaunt’s daughter from his 2nd marriage (Catherine of Lancaster) was married to King Henry III of Castile, Gaunt was the grandfather of King John II of Castile. Inasmuch as John of Gaunt was the maternal grandfather of King John II, he is actually the “ancestor of all subsequent monarchs of the Crown of Castile and united Spain”. PLUS, through John II of Castile’s great-granddaughter Jaunna the Mad, “John of Gaunt is also an ancestor of the Habsburg rulers who would reign in Spain and much of central Europe.”
In sum, if you were a Medieval Monarch in Europe after the 14th century, you could trace your ancestry back to John of Gaunt … and if you are a modern royal, you STILL can!