At a young age, Théodore Garnier participated in the Franco-Prussian War of 1870–71 as a Pontifical Zouave under General de Charette.

[2] A talented and provocative preacher, he took the side of workers, founding a job placement office and a people's bank while advocating for a corporatist inspired Christian association of industry and trades.

Appointed "apostolic missionary" by Pope Leo XIII, he focused on bridging the gap between the Catholic Church and the working class.

His sermons attracted large audiences but also provoked disorder and clashes with anticlerical activists, such as the riot at the Rouen Cathedral in January 1888.

[5] Théodore Garnier founded several institutions, including the Union Nationale in 1892 and a Catholic workers' club near Notre-Dame de Clignancourt in Paris.