In 1520, political frictions with Charles V convinced his father, Henry I of Duke of Brunswick-Lüneburg to abdicate and leave for the French Court which was ardently Catholic.

Political considerations furthered the introduction of the Reformation; amongst the commoners it offered opportunity to restrict the privileges of the nobles and the clergy.

After an attempt by the Roman Catholic party in 1527 to reinstate his father had failed, Ernest's course became more decided as he succeeded as Duke.

At a diet in August of the same year it was ordered that "God's pure word should be preached everywhere without additions made by men."

The largest and richest monastery in the land, St. Michael's in Lüneburg, accepted the new order after the death of Abbot Boldewin in 1532.

In the cities of Westphalia he strengthened the Protestant party against both the Roman Catholics and the enthusiasts, although his efforts were vain in Münster.

He induced the North German cities, Hamburg, Bremen, Brunswick, Göttingen, and others to join, and he often became the successful mediator when a rupture was threatened between the overcautious elector of Saxony and the headstrong Philip of Hesse.

Ernest married Sophia, daughter of Henry V, Duke of Mecklenburg and Ursula of Brandenburg, on 2 June 1528 in Schwerin.