Who is the duke of Lancaster? It’s a simple answer that requires a not-so-simple explanation.
The first creation of the dukedom of Lancaster was on the 6th of March 1351, for Henry of Grosmont, 4th Earl of Lancaster, a great-grandson of Henry III. Like many of his ilk, he wore lots of other hats. He was also 4th Earl of Leicester, 1st Earl of Derby, 1st Earl of Lincoln and Lord of Bowland. Let’s face it, this was an important person, holding a great deal of land and wielding oodles of power. He was as close to the throne as could be. He was a favorite captain of Edward III (Richard II’s grandfather) and a founding member of the Order of the Garter. He distinguished himself well in many different ways.
How John of Gaunt became duke on the 13th of November, 1362, was after he married his cousin and Henry of Grosmont’s daughter, Blanche of Lancaster, 6th Countess of Lancaster, and the old duke died. John of Gaunt, 1st Earl of Richmond, was the fourth son of King Edward III. He certainly wasn’t going to inherit the throne (although in those days with battle and disease, you never knew), so John set to the task of diplomat, soldier, and all around rich guy, the richest in England, in fact. I put my fictional detective, Crispin Guest, into his household as the perfect place for a knight to grow to maturity and to understand the ways of chivalry. John was a knight of knights,right enough, and Crispin couldn’t have done better. Geoffrey Chaucer also lived and worked in that household so it was a perfect fit making him Crispin's best bud as well. (You can read more about John of Gaunt here.)
When John of Gaunt died on February 4, 1399, the Dukedom passed to his son, Henry of Bolingbroke, Earl of Derby and 1st Duke of Hereford, though at this time, Richard had had him banished. Henry had already been part of a group of clergy and noblemen who, with armed soldiers, forced King Richard to do right by Parliament and quit spending the treasury on so many favorites (See BLOOD LANCE and SHADOW OF THE ALCHEMIST). But Richard rose up in power again and got his cousin out of the way. Later that same year, when Henry returned with an army, he essentially usurped the throne of England away from Richard II, took the throne himself as Henry IV, and merged the Dukedom with the crown, and decreed that it would be personally inherited by the monarch.
And so the present Duke of Lancaster is Her Majesty Queen Elizabeth II, who is “the Duke,” by the way, not the Duchess. Further, it is customary today at formal dinners in Lancashire and in Lancastrian regiments of the armed forces for the Loyal Toast to the crown to be announced as "The Queen, Duke of Lancaster." The Duchy remains and is, in fact, a good source of income for the Queen. You can read more about that here.