His father, Christian II, had ruled jointly with his uncle, Otto I.
After Christian II died in 1233, Otto I acted as guardian for the underage John I.
In 1258 and 1259, he fought in the Bremen Prince-Archbishopric Feud on the side of his cousin Hildebold of Wunstorf against Rüstringen, Östringen and Stedingen.
As a replacement, John I and his uncle built a castle in Delmenhorst, which provoked a strong reaction from Stedingen.
He married Richeza, a daughter of Count Henry II of Hoya, and had the following children: