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The Landgraviate of Hesse-Kassel was formed in 1567 following the division of the Landgraviate of Hesse between the four sons of Landgrave Philip I. The eldest son, William IV, inherited Hesse-Kassel. Hesse-Marburg was given to Louis IV and Philip II became the Landgrave of Hesse-Rheinfels. The youngest son, George, received the Hessian lands in the former County of Katzenelnbogen which became Hesse-Darmstadt. Hesse-Kassel was a state of the Holy Roman Empire until its dissolution in 1806. In 1604, Hesse-Kassel was expanded when Langrave Maurice inherited the Landgraviate of Hesse-Marburg upon the death of Landgrave Louis IV. He did not have an heir and the land was received by Hesse-Kassel. This sparked a rivalry between Hesse- Kassel and Hesse-Darmstadt. In 1065, the rivalry was fuelled further as Hesse-Kassel became Calvinist and Hesse-Darmstadt became Lutheran. In 1065, Hesse-Kassel entered the Thirty Years’ War on the Protestant side. Maurice was forced to cede some of the territory of Hesse-Kassel to Hesse-Darmstadt and abdicated in favour of his son William V in 1627. In 1627, Hesse-Darmstadt and Hesse-Kassel reached an agreement, but it was quickly revoked and resulted in the Siege of Dorsten.Ten years later, William V died in exile, leaving his wife, Amalie Elisabeth of Hanau-Münzenberg, to act as regent for William VI, their eight-year-old son. From 1645, Hesse-Darmstadt and Hesse-Kassel were engaged in a series of battles against each other and was settled on the eve of the 1648 Peace of Westphalia. As a result of the Peace of Westphalia, Hesse-Kassel was granted most of the County of Schaumburg and Hersfeld Abbey. The rule of primogeniture was introduced by Amalie Elisabeth to prevent Hesse-Kassel from being divided again in the future. She died in 1651.

William VI succeeded Amalie Elisabeth and was an enlightened patron of the arts and of learning. He was succeeded by his son, William VII, in 1670. William VII was then succeeded due to his death in the same year by his brother Charles I. Charles hired out his soldiers to foreign powers as auxiliaries, which improved the finances of the principality. The next Langrave was Frederick I of Sweden, who became King of Sweden through marriage. From 1730 to 1751, Hesse-Kassel was in personal union with Sweden and Prince William ruled as regent until he succeeded Frederick as William VIII in 1751. During the Seven Years’ War, Hesse-Kassel was an ally of the Kingdom of Great Britain. Frederick II, his successor, converted to Catholicism after a long line of Protestant Langraviates. During the American Revolutionary War, Frederick II hired out Hessian soldiers to Great Britain for service in America. The Reichsdeputationshauptschluss of 1803 authorized the Langraviate of Hesse-Kassel to be raised to the Electorate of Hesse and Landgrave William IX was given the title of Imperial Elector, becoming the Elector of Hesse Wiliam I. The principality became known as Kurhessen, although it is still referred to as Hesse-Kassel. During the dissolution of the Holy Roman Empire and the War of the Fourth Coalition, William I was dispossessed by Napoleon for his support of Prussia. Hesse-Kassel was appointed the capital of the new Kingdom of Westphalia and Napoleon appointed his brother, Jérôme Bonaparte, as king. In 1813, Napoleon was defeated and the elector was restored. At the Congress of Vienna in 1815, several Napoleonic electorates were elevated to kingdoms. William chose the title of Prince-Elector. The only Prince-Electors in the German Confederation were the rulers of the Electorate of Hesse.

On February 27, 1821, William I died and was succeeded by his son, William II. Under his rule, the crisis in Hesse-Kassel grew. His wife was a popular Prussian princess who was adored by the people, but William II’s mistreatment of her made him extremely unpopular. The July Revolution in Paris forced William II to summon the Estates. He retired to Hanau and his son, Frederick William was appointed as regent. Frederick William joined the Prussian Northern Union after the breakdown of the Frankfurt National Parliament. Deputies from Hesse-Kassel were sent to the Erfurt Parliament. However, the Elector’s policies changed after Austria regained strength. The state was annexed by Prussia after the Austro-Prussian War in 1866.

During World War II, allied air raids destroyed large parts of the city of Kassel. In April 1945, United States troops occupied the city. In 1949, Kassel applied as the new capital of the Federal Republic of Germany, but the motion was defeated.