In 1919 he was part of the counter-revolutionary government (firstly formed in Arad) under the leadership of Count Gyula Károlyi, which was established to oppose the Hungarian Soviet Republic. In this government he was Minister of Foreign Affairs from 5 May to 31 May. Along with other members of the government, Bornemisza was interned by Romanian authorities to Mezőhegyes between 9 May and 22 May, during the Romanian intervention in Hungary.