At the time of his father's death in 1434 Wenceslaus I was already grown enough to assume the government of the Duchy by himself and to take the guardianship of his younger brothers Przemysław and Jan IV.

For unknown reasons he did not attend the Congress of Będzin on 15 October 1434, where, among other things, were defined the frontiers between Lesser Poland and Silesia; which more surprising that there was present his stepmother Margareta.

The next step was done on 8 January 1441, when Wenceslaus I was compelled to pay homage to the Polish king and a further diminution of his power in the government on behalf of his younger brothers.

The common rule of Casimir I's sons over Oświęcim lasted until 19 January 1445, when, at the request of Duke Nicholas V of Karniów, the duchy was divided into three separate sections: Jan IV took Oświęcim, Przemysław obtained Toszek and Wenceslaus I received Zator; it is unknown why Wenceslaus I, as the oldest son, didn't receive the main town of the Duchy and obtained the relative small town of Zator.

As a duke of Zator, Wenceslaus I tried to lead a policy of approaching to Poland, despite his still existing ties with the Bohemian Kingdom (he was formally a vassal of Bohemia).