Mandić was living as a retired government functionary at the time the Independent State of Croatia ("Nezavisna Država Hrvatska", NDH) was declared. He worked as an attorney and served as president of the Sarajevo Chamber of Attorneys until September 1943. On 2 September 1943, "Poglavnik" Ante Pavelić offered Mandić the post of Prime Minister of the Independent State of Croatia. Mandić accepted the offer. His appointment was met with mixed feelings, and infuriated politicians such as Mladen Lorković, Mile Starčević and Vladimir Košak, some of whom threatened to resign due to the decision. They brought up Mandić's age and questioned whether he would be able to serve to the best of his ability.