Pius August was a grandfather of Empress Elisabeth of Austria through his son Duke Maximilian Joseph in Bavaria, as well as a great grandfather of Queen Elisabeth of the Belgians and an ancestor of the current generations of the Belgian and Italian Royal Families and the Grand Ducal Family of Luxembourg.

Pius August was born on 1 August 1786 in Landshut as the third child and eldest surviving son of the then Count Palatine Wilhelm of Birkenfeld-Gelnhausen and his wife, Countess Palatine Maria Anna of Zweibrücken-Birkenfeld.

His father was a member of the non-reigning Palatinate-Birkenfeld-Gelnhausen line of the widely branched House of Wittelsbach, and his mother was a daughter of Count Palatine Frederick Michael of Zweibrücken-Birkenfeld-Bischweiler, and sister of Maximilian Joseph, later King of Bavaria.

[2] Pius August married Princess Amélie Louise of Arenberg, daughter of Prince Louis Marie of Arenberg (1757-1795) and his wife, Marie Adélaïde Julie de Mailly-Nesle, Dame d'Ivry-sur-Seine (1766-1789), on 26 May 1807 in Brussels.

[3] In 1815, Pius August became an honorary member of the Bavarian Academy of Sciences and Humanities.