As a high school student, he began to publish poems, reviews, articles and debates in several Croatian newspapers and magazines.
In his autobiography, Mortigjija said that in his youth he was a keen Yugoslavian, but his opinion changed in 1928 after Puniša Račić's attack in the Yugoslav Parliament.
Beginning with the establishment of the Independent State of Croatia in April 1941, Mortigjija was editor of the most important national newspaper, Hrvatski narod.
In February 1942 he left the newspaper to work as director and chief editor of the newly founded weekly magazine Spremnost, a journal of the Ustaše movement.
They published anti-Jewish propaganda which quoted text from Deutsche Zeitung—German language newspapers printed by the Nazi Party through Franz Eher Nachfolger.
While in the refugee camp, Mortigjija accepted a job to organize social activities, events, and lectures on Croatian history.
Mato Mortigjija's lawyer, Antun Mihočević, argued a long trial at the County Court in Zagreb.
[5] The verdict was greeted in some emigrant circles as a rare example of correcting injustice committed during by the communist Yugoslavian government.