William Otto was born at Dillenburg Castle[1][note 1] on 23 June 1607[1][2][3][4] as the third son of Count John VII 'the Middle' of Nassau-Siegen and his second wife, Duchess Margaret of Schleswig-Holstein-Sonderburg.

[7][8] After his older half-brother John 'the Younger' had accepted the homage of the city of Siegen for the entire county of Nassau-Siegen on 12 January 1624[9] and had voluntarily ceded the sovereignty over the Hilchenbach district with Ginsburg Castle [de] and some villages belonging to the Ferndorf [de] and Netphen districts to his younger brother William on 13/23 January 1624,[10][11] William Otto and his younger brothers accepted only modest appanages.

[12][13] During the Thirty Years' War William Otto served in the Swedish Army[2][3][14][note 3] under Duke Bernhard of Saxe-Weimar.

[2] He was killed in a cavalry battle[2][3][14] near Wolfenbüttel on 14 August 1641,[1][note 4][2][3][4] and was buried in Kassel on 16 September 1641.

[2] William Otto was the only one of the many sons of Count John VII 'the Middle' who never served the Dutch Republic.