Mandić was born in Lise, Herzegovina (present-day Široki Brijeg). He completed his primary education in Široki Brijeg, where he attended the famous Franciscan high school, but graduated from the last two years in Mostar. He studied theology at Fribourg and obtained his PhD in church history. When he returned to Mostar, he became a teacher of religion in the Mostar state high school. The Franciscan Province of Herzegovina elected him as their head.
In 1939, Mandić was appointed a member of the central administration of the Franciscan Order in Rome as the representative of all the Franciscan provinces in Slavic countries, and Chief Economist of the Order. He was in Rome when war broke out in the Balkans in April 1941. As Chief Economist and General Definitor of the Franciscan Order, he provided financial support to the Ustaše, as Mark Aarons and John Loftus reference in Unholy Trinity: The Vatican, The Nazis, and The Swiss Banks.
Mandić controlled San Girolamo ratline's finances. He arranged the laundering of Ustasha money likely via the Franciscans' Vatican Bank accounts to which he had access and placed the Franciscan printing presses at the disposal of the Ustasha to print false identity information for war criminals to escape from justice after the Holocaust using ratline escapes. Other priests involved in the San Girolamo ratline included: Krunoslav Draganović, Dragutin Kamber, Vilim Cecelja (based in Austria) and Karlo Petranović (based in Genoa).
- Hrvati i Srbi, dva stara različita naroda (Croats and Serbs, Two Ancient and Different Peoples)
- Bogumilska crkva bosanskih krstjana (The Bogumil Church of Bosnian Christians)
- Crvena Hrvatska (Red Croatia)
- Državna i vjerska pripadnost sredovječne Bosne i Hercegovine (State and Religion in Medieval Bosnia and Herzegovina)
- Etnička povijest Bosne i Hercegovine (Ethnic History of Bosnia and Herzegovina)
- Mark Aarons & John Loftus, Unholy Trinity: The Vatican, The Nazis, and The Swiss Banks, St. Martin's Griffin (revised edition, 15 June 1998), page 118; ISBN 978-0312181994
- The Collected Works of Dominik Mandić, fram.ba (PDF)