He was born as Jeroom Gustaaf De Clercq in Everbeek, East Flanders on 16 September 1884.

He was a member of the moderate Frontpartij, became party leader in 1932, and moved them to the right, converting them into VNV the following year.

Welcoming of the Nazi German occupation, De Clercq believed it to constitute a chance for the creation of a Diets state, an unprecedented (apart from the medieval Burgundian personal union) Dutch language-based community uniting Flanders, the Netherlands and even the part of northern France with Flemish dialects (corresponding to French Flanders).

His organization supported the German occupiers in the identification and round-up of Jews for deportation.

He remains revered among some far-right Flemish nationalists in modern Belgium.