After Władysław's death between 1321 and 1324, Jan succeeded him in Oświęcim and in consequence was forced to leave his spiritual career.

During the first year of his reign (1324–1325), he was placed under the regency of his mother, the Dowager Duchess Euphrosyne, who remained involved in the government of the duchy until her death, in 1329.

On 24 February 1327, together with the other Piast Dukes, Jan paid homage to King John of Bohemia in Opava.

In 1355 Jan participated in the congress in Prague, where a dispute was resolved between the Dukes of Cieszyn and Oleśnica for the division of the Duchy of Bytom.

His generosity to the Church was recognized by the Provincial Chapter of Płock in 1372, when prayers for the soul of the deceased Duke were ordered in the whole country.