Mandić became a member of the Diet of Bosnia ("Bosanski sabor") in 1910, representing the HNZ. He was elected Speaker of the Diet in 1911 and was named vice-governor of Bosnia and Herzegovina by decree of Emperor Franz Joseph. Mandić was also the founder and first president of the Croatian Central Bank ("Hrvatska centralna banka", HCB), as well as its subsidiary, the Agricultural Bank of Sarajevo ("Poljoprivredna banka u Sarajevu", PBS). He also founded a Croatian choir called "Trebević". Mandić remained a member of the Bosnian Diet until Austria-Hungary's dissolution in November 1918.