Mandić was born on 4 April 1883 in Trieste, Austria-Hungary (today in Italy).
He also attended the Vienna Conservatory, taught by Hermann Graedener and Robert Fuchs.
Mandić returned to native Trieste, opened a law office there, and became involved in politics as a political ally of Dinko Trinajstić.
[1] During the World War I, he became a member of the Yugoslav Committee, an ad-hoc group of politicians and activists advocating unification of the South Slavs and dissolution of Austria-Hungary.
[2] After the war, Mandić stayed in Switzerland until 1923 when he moved to Prague, Czechoslovakia where he practised law and wrote music again.