In both 1390 and 1397, John and his siblings were declared legitimate by Parliament, as well as by the Pope in September 1396, after his parents were married. Despite being made legitimate by both the church and state, Henry IV barred John and his Beaufort siblings from succession to the throne, even though they too were the grandchildren of Edward III.
Between May and September 1390, John Beaufort embarked on the Barbary Crusade, led by Louis II, Duke of Bourbon. The objective of this crusade was to curtail piracy in and around Mahdia; however, the siege proved unsuccessful.
In 1394, John Beaufort served in Lithuania with the Teutonic Knights.
On 10 February 1397, John was created Earl of Somerset. Also in February 1397, he also was appointed constable of Dover Castle, warden of the Cinque Ports, and admiral of the Irish fleet. In May 1397, John Beaufort’s admiralty was extended to include the northern fleet.
In the summer of 1397, John Beaufort helped Richard II extricate himself from the power of the Lords Appellant. As a reward, John Beaufort was married to Margaret Holland, niece of Richard II on 27 September 1397. (Together, John Beaufort and Margaret Holland had six children.)
On 29 September 1397, John Beaufort was named Marquess of Somerset and Marquess of Dorset.
In late 1397, John Beaufort was honored as a Knight of the Garter and was appointed Lieutenant of Aquitaine.
In 1398, Richard II banished John Beaufort’s half-brother Henry Bolingbroke (a.k.a. Henry IV) from England; however, John Beaufort remained in Richard II’s good graces.
In 1399, Henry Bolingbroke came back to England and deposed Richard II. As the reigning king, Henry IV rescinded the titles Marquess of Somerset and Marquess of Dorset;, making John Beaufort only Earl of Somerset again. Despite this demotion, John Beaufort remained loyal to Henry IV.
In 1404, John Beaufort was named Constable of England.
Then, at the age of 37, John Beaufort died on 16 March 1410, in the Hospital of St Katharine’s by the Tower. He was buried in St Michael’s Chapel in Canterbury Cathedral.