John Beaufort, First Earl of Somerset, was born to John of Gaunt, Duke of Lancaster, and Katherine Roet. His parents were lovers, married to others, and they did not marry until much later in life. Therefore, John and his siblings were illegitimate.

However, in 1397 his birth was legitimized by both Parliamentary and Papal decree. He was created First Earl of Somerset in 1397, as well as at that time created First Marquess of Somerset and First Marquess of Dorset. He served as the Constable of England in 1404. Despite being a direct descendant of Edward III (he was Beaufort's grandfather), by agreement he was barred from the succession to the throne.

He was a military man, serving with Louis II's crusade in North Africa. A few years later he traveled to Lithuania and served with the Teutonic Knights. He was instrumental in helping Richard II overcome the powerful Lords Appellant, and was rewarded with the Marquess of Dorset title. Later the same year he was made a Knight of the Garter and appointed Lieutenant of Ireland.

He married Lady Margaret de Holand, niece of the king and sister of the Third Earl of Kent. The couple went on to have six children, and it is through this line that the Tudor family descends. Beaufort continued to remain in the king's good favor even after his half-brother, Henry Bolingbroke (later Henry IV) fell from grace and was exiled by the king. The king also appointed Beaufort Admiral of the Irish fleet, and two positions of constable. However, once Richard II was deposed by the new King Henry IV, Beaufort lost a number of his titles, but was left with the Earl of Somerset designation. However, he retained the favor of his half-brother and served in a variety of honorable positions, and was awarded important estates in Wales.

Beaufort died in March of either 1409 or 1410 at the Hospital of St. Katherine-by-the-Tower in the City of London, and was buried at Canterbury Cathedral.