His uncle Charles, Duke of Södermanland (1550–1611), the closest adult in the line of succession, took up the regency, and, until 1604, no king was proclaimed.

Instead, he received promises of several duchies, including northern and western Östergötland, Kinda and Ydre districts in Småland, the county of Läckö, and the entire small province of Dalsland.

At the parliament of Nyköping the same year, Gustav II Adolf was recognized as the king, and Duke John renewed his renunciation.

On 29 November 1612, Duke John married his first cousin, Princess Maria Elizabeth of Sweden (10 March 1596 – 7 August 1618), daughter of Charles IX, at Tre Kronor Castle in Stockholm.

He draw guidelines for the new Saltängen area of the town and founded Holmens Bruk, where arms manufacturing started.

Aside from John's childless widow, his elder half-brother King Sigismund of Poland and half-sister Anna of Finland survived him and were his closest relatives.