William was born in Dillenburg on 13 August 1592,[1][note 1] the fifth son of Count John VII 'the Middle' of Nassau-Siegen and his first wife, Countess Magdalene of Waldeck-Wildungen.

[11] Accompanied by 24 companies of infantry and some cavalry, William was sent to the Bommelerwaard by Prince Frederick Henry of Orange to prevent an invasion by Hendrik Graaf van den Bergh, then commander of the Army of Flanders.

[4] In the winter of 1629–1630, William was sent with his troops east of the Rhine to Cleves, Mark, Jülich and Berg, but without providing sufficient funds.

The prince argued in response that it were vrembde maximen ('strange maxims') to worry about this, because after all die Landen den Vurst van Nieuburch toebehoorende waeren, ende dat het Volck alleene gesonden was om desen winter aldaer geïnquartiert te worden ('the lands belonged to the Prince of Neuburg, and the troops had only been sent to be quartered there this winter').

In Soest Walraven van Gendt also complained but the presence of these soldiers forced the Spanish troops to withdraw from Unna, Hamm and Lippstadt, leaving them to the Dutch.

[1][3] During the March along the Meuse [nl] in 1632, William and his troops initially kept an eye on the situation near the front in Zeeland until he was summoned by Prince Frederick Henry of Orange to the Siege of Maastricht in late July.

[14] William captured the Kruisschans on the Scheldt and Fort Sint-Anna [nl] and other sconces near Antwerp, later Orsoy, in the following year the Sterreschans and those of Philippine in State Flanders [fr].

[7] In 1632 William was appointed ritmeester of the cuirassiers[3] and in April 1633 succeeded his uncle Count Ernest Casimir of Nassau-Diez as Field marshal.

[4] In support of his planned attack on Antwerp, in 1638 Prince Frederick Henry entrusted William with an important undertaking, the occupation of the levee at Calloo.

Receiving false intelligence that a larger Spanish force was approaching, he hastily retreated but was caught and defeated at the Battle of Kallo on 17 June.

[21] Only when John 'the Middle' was convinced that his son was under the influence of the Jesuits and that the possibility of a Catholic area within the Nassau lands was a danger to the Protestant inhabitants, was he persuaded to make a new will.

[19] On 3 July 1621 John 'the Middle' drew up a third will, in which he laid down that the small county of Nassau-Siegen, which was barely able to support one lord, was to be split into three parts.

He had taken the precaution of obtaining a further imperial decree on 20 November 1623 against Countess Dowager Margaret and her sons, in which the Emperor strictly forbade impeding John's assumption of government, his taking possession of the land and his inauguration.

For a short period (1632–1635) this situation underwent a temporary change, during the Thirty Years' War, his brothers, who were fighting on the Protestant side, rebelled against John 'the Younger'.

[22] Count Louis Henry of Nassau-Dillenburg entered the service of King Gustavus II Adolphus of Sweden on 1 December 1631, who had landed in Germany on 24 June 1630 to intervene in favour of the Protestants in the Thirty Years' War.

[27] Countess Dowager Margaret, through the mediation of Louis Henry, turned to Gustavus Adolphus and asked for help against the machinations of her stepson John 'the Younger'.

On 14 February 1632 the Swedish king sent an order from Frankfurt to Louis Henry to provide military support for his first cousin John Maurice.

The latter soon restored the old order: in 1636, he again became the sole owner of his father's property, with the exception of Hilchenbach, which he left to William and he again governed the city of Siegen alone.