Rousseau was born in Nancy in 1969, the son of Claude Rousseau, an engineer and an officer of the Order of Leopold II of Belgium.[3]
He made his solemn vows as a full Knight of Justice on 11 May 2011 at the church Sainte-Élisabeth-de-Hongrie in Paris.[4] A Knight Grand Cross of Justice, he was elected to a five-year term as a member of the Sovereign Council in May 2014.[5] He was elected to a second five-year term as a member of the Sovereign Council in May 2019.[6] That term ended when Pope Francis reorganized the leadership of the Sovereign Order of Malta on 3 September 2022 and appointed him Grand Commander of the Order of Malta.[7] On 26 January 2023, he was elected to a six-year term in that office.[8]
Rousseau is curator of the Magistral Libraries and Archives[9] and Vice-President of the (French) Académie historique de l'Ordre de Malte.[10]Dominique, Prince de La Rochefoucauld-Montbel, who was President of the French Association and Grand Hospitalier of the Order of Malta is now the President of the Académie.
With Étienne Martin and Bruno Martin: Saints et Bienheureux de l’Ordre de Malte, 2014.[12]
With Laure Peureux, Olivier Cubourg, and Hugues Lefort: "L’Ordre de Malte pendant la Première Guerre mondiale", Soins 59, no. 786 (juin 2014): 80–82.[13]
Introduction to Valeria Vanesio, Il valore inestimabile delle carte: l'archivio del Sovrano Militare Ordine di Malta e la sua storia: un primo esperimento di ricostruzione. Roma: Tip. Mariti, 2014.
"La vie spirituelle du Cardinal Tisserant", Colloque Le Cardinal Tisserant au service des Églises orientales, 21 octobre 2017, église Sainte-Élisabeth-de-Hongrie, Paris.[14]