His real name was Joannes Alphonsius Albertus Goris; his pseudonym relates to Marnix van Sint Aldegonde and the surname of his mother (Gijsen).

In his youth he received a strict Roman Catholic education, at the Jesuit college of Saint Ignacio in Antwerp, but in 1917 he was punished heavily (consilium abeundi, E: 'advice to leave') for his militant Flemish activism during World War I.

Marnix Gijsen started his literary career as a poet within the expressionist group Ruimte (Space) their illustrated magazine.

During World War II, he broke with his Roman catholic faith, and adopted moral values and an attitude of life based on stoicism.

On his relation to Catholicism, he wrote De afvallige (The renegade) and Biecht van een heiden (Confession of a heathen), which were both published in 1971.