Mahnič was born in Kobdilj near Štanjel in the Austrian County of Gorizia and Gradisca (in today's Slovenia).
In his bishopric he initiated many religious societies and activities, and started Catholic publishing, including a magazine for Christian philosophy called Hrvatska straža.
He wrote many articles and works in the theological, philosophical, esthetical and political area.
After the war, Mahnić was persecuted after the territory was annexed by Italy, and he went to Zagreb, where he died.
Mahnić's main goal was defending and promoting Catholic faith and its moral principles in Croatian public and social life, which were endangered by liberalization and secularization.