Vas county arose as one of the first comitatus of the Kingdom of Hungary.
In 1918 (confirmed by the Treaty of Trianon 1920), the western part of the county became part of the new Austrian land Burgenland, and a smaller part in the southwest, known as Vendvidék became part of the newly formed Kingdom of Serbs, Croats and Slovenes (Yugoslavia). In the Vendvidék in 1919 was founded an unrecognized state the Prekmurje Republic, alike in Burgenlad the Lajtabánság. The remainder stayed in Hungary, as the present Hungarian county Vas. A small part of former Sopron county went to Vas county. Some villages north of Zalaegerszeg went to Zala county, and a small region west of Pápa went to Veszprém county.