He initially reigned jointly with his brother, Albert the Tall, until the partition of the duchy in 1269, when John became the first ruler of the newly created Principality of Lüneburg.
John's father, Otto the Child, was the first Duke of Brunswick-Lüneburg, having received the Welf allodial possessions in Saxony from the hands of Emperor Frederick II.
In a fierce feud with Count Gunzelin III of Schwerin, John was able to acquire the settlement of Uelzen, whose residents he vested with town privileges in 1270.
His son and heir, Otto II the Strict, was still a minor upon his father's death and remained under the tutelage of his uncles Duke Albert the Tall and Bishop Conrad of Verden until 1282.
The marriage produced had five children: His illegitimate son, Henry of Brunswick (d. 23 August 1324), was a canon in Walsrode.