The medieval core of the city, the Castle of Vác, was a fortification built on a small hill near the river bank. This is where King Géza built a cathedral dedicated to the Virgin Mary, and he was buried there in 1077.
Settlement in Vác dating as far back as the Roman Empire has been found.
Bishops from the diocese were influential within the Kingdom of Hungary, with many serving as chancellors or later becoming archbishops.
The town was conquered by the Ottoman Empire in 1541. During the Habsburg Monarchy's wars against the Ottomans, the Austrians won victories against the Turks at Vác in 1597 and in 1684.
During the Hungarian Revolution and War of Independence of 1848-49, the Honvédség routed the Austrian forces stationed in the city after a major battle (April 10, 1849); the Second Battle of Vác ended in Russian victory (July 17).