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.
[citation needed] In the Kingdom of Serbs, Croats and Slovenes (established in 1918), Mandić was a member and a supporter of the pro-Yugoslav Croatian Popular Party (HPS).
Mandić, unlike other HPS members, supported Barić's stay in the government, for which he was decorated with the Order of Saint Sava 2nd class by King Alexander I.
Immediately after the German invasion of Yugoslavia in April 1941, and after the Yugoslav government left in exile in London, Mandić established contacts with London through his contacts in Switzerland and entered a cooperation with the British Secret Intelligence Service.
[3] Upon hearing rumors about the deportation of Serbs and Slovenes in the Independent State of Croatia (NDH), Mandić tried to influence the Government of the Independent State of Croatia to stop the deportations and to prevent clergy to participate in such doings.
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.