Mortier studied painting and sculpting. In 1940 he began his artistic career, next to his professional career to make a living. In 1944 and 1945 he exhibited at the Salon Apport, organised by Robert Delevoy and was a member of the Jeune Peinture Belge for a short period of time (1945-1946). He is being seen as one of the more important representatives of lyric abstraction in Belgium. His broad gestures on paper or on canvas brushed works are inspired by his daily surroundings, in which he transcends the themes, looking for a formal balance, which is the expression of intense emotions.
Mortier used oil-paint as well as East-Indian ink. Black plays an important role and often goes hand in hand or contrasts with a pallet of bright colours. He often worked on a monumental size which was fit for a broad gesture. The oeuvre of Mortier radiates a great power and at the same time it reveals the inner truth of a man looking for the absolute.