In 1312 John and his paternal half-brother Gerhard IV succeeded their father as co-ruling counts of Holstein-Plön.

A wealthy man by inheritance John very early acted as a powerful local prince funding Danish warfare and co-operating with his first cousin Gerhard III.

His financial support of the Danish kings quickly made him one of their leading creditors with his chief possession in Funen.

After Gerhard III's death and the collapse of Holstein-Rendsburg's rule in 1340, John at first co-operated with King Valdemar IV in order to get his money back but during the following years all his Danish possessions were lost through wars and unfavorable economic transactions.

In fact not much is known about John’s character but he seems to have been a clever diplomat whose ambitions owing to circumstances were gradually limited to economic demands.