After a grand tour and studies at the universities of Strasburg, Basel and Geneve, he was made administrator of the abbey of Hersfeld in 1617. In 1619 he married Countess Amelia Elisabeth von
Hanau-Muenzenberg and they became the parents of twelve children. A member of the Protestant Union, Hessen-Kassel suffered a great deal during the early years of the Thirty Years War. The
principality's Lutheran estates refused Wilhelm's Calvinist father the necessary finances to defend their country against the Catholic League. Moritz, Wilhelm's father, also had problems with neighbouring Hessen-Darmstadt whose Landgraf, even though a Lutheran, sided with the Emperor.

In 1623 this dispute with Hessen-Darmstadt was settled in favour of Hessen-Darmstadt by the Imperial Aulic Council. This granted all of Upper-Hessen to Darmstadt. Count Tilly, with his Catholic forces, not only occupied most of Hessen-Kassel but also pursuaded the country's
nobility both to remain neutral and not assist Moritz with defending their country against the imperial forces.
In 1627 Moritz abdicated in favour of Wilhelm V who aimed to stabilize and strengthen his country. To achieve this, he signed a treaty with Hessen-Darmstadt and reorganized his country's
administration and army, through these efforts becoming a leader in the more forceful Protestant defence.
In 1631 King Gustavus Adolphus of Sweden entered the war and Wilhelm V was one of the first to join him. Even before the Swedish victory at Breitenfeld, Wilhelm V's army had started to liberate
Hessen-Kassel from the Catholic forces. Once Hessen-Kassel was freed, his forces then invaded Westphalia and assisted the Swedes with the siege and subsequent conquest of Mainz.

Even though he supported the Swedish-dominated Heilbronn League, he did not himself join it formally. After the Swedes were defeated at Noerdlingen in 1634, he began looking to France for financial and military aid. In 1635 he was one of the few German princes who did not accept the peace of Prague, objecting because it excluded Calvinists. In 1636 he signed an alliance with France and continued his war against the Emperor.
In response, Emperor Ferdinand placed Wilhelm V under the imperial ban and, when he invaded Hessen-Kassel, Wilhelm V was forced to flee with his family. While at his military headquarters at Leer in East-Friessia, he fell ill and died there on 21 September 1637 at only thirty-five years of age.
 

Source: Leo van de Pas