Defaux was born in Gaurain-Ramecroix, Tournai, on April 20, 1860.[3] He was first a stonemason and soon became involved in trade union activity, trying to organize the working class. For this reason, he had a hard time dealing with the local employers and had to look for another job, which he found during the construction of the Edingen-Ronse-Vloesberg railway.[2] He nevertheless became the first permanent secretary of the stonemasons' union in Tournai and remained so until 1918.
In his native village he became a municipal councilor (1904), alderman (1908) and mayor (1912–1920). He was also a provincial councilor for Hainaut (1912–1918).[1]
In 1918 he succeeded Émile Royer, who died during the World War I, as a socialist representative for the Tournai – Aat district. He then exercised this mandate until his death.[2] He remained unmarried.
In Gaurain-Ramecroix a monument was erected in his memory and there is a Résidence Joseph Defaux.