From 1611 to 1621 John Albert and his brother Adolf Frederick I jointly ruled the whole Duchy of Mecklenburg.

He reigned from 16 April 1608, under the regency of his great-uncle Duke Charles I, jointly with his brother Adolf Frederick I in the Mecklenburg-Schwerin part of the country.

They tried to seem neutral during the Thirty Years' War, but they secretly supported the Danish troops of king Christian IV.

On 19 January 1628, Emperor Ferdinand II issue a decree at Brandýs Castle declaring that the brothers had forfeited their fief and that Mecklenburg would be invested to Wallenstein.

(I) On 9 October 1608 he married Margaret Elizabeth (11 July, 1584 – 16 November, 1616), daughter of Duke Christopher of Mecklenburg.