Influenced by Franz Kafka, Mertens started to publish novels and short stories in 1969 and received the Prix Médicis in 1987 for Les éblouissements.
Mertens was particularly marked by the activities of his parents, his father a journalist and music lover and his mother a biologist and pianist.
He was also much affected by the German occupation, the execution of the Rosenbergs and the tragedy of the miners of Marcinelle in 1956.
Later as a scholar of international law he denounced the genocide in Biafra, torture in Ireland, and the prisons of Pinochet.
In Les Bons offices (1974) and Terre d'asile (1978) Belgian history is presented from a foreign perspective.