The hereditary Grand Duke Guillaume was born on 11 November 1981, and as of this year he is the official lieutenant representative to his father, the Grand Duke.

The hereditary Grand Duke Guillaume is the eldest child of Grand Duchess Maria Theresa and Grand Duke Henri. Nominated as the official successor to the Grand Duke on 18 December 2000, he married the Countess Stéphanie de Lannoy on 19 October 2012.

On 10 May 2020, the prince welcomed his first child, Charles Jean Philippe Joseph Marie Guillaume, born at 5.13am at the Grande-Duchesse Charlotte maternity hospital in Luxembourg City. His younger brother, Prince François Henri Louis Marie Guillaume, was born on 27 March, 2023.

Prince Guillaume was officially appointed as Lieutenant-Representative of his father, Grand Duke Henri, on 8 October 2024, during a ceremony at the Grand Ducal Palace. This role allows him to assume many of the Grand Duke's constitutional duties.