In 1456 the cathedral chapter of Trier elected John II of Baden against Diether.

On 18 June 1459 he was elected the Archbishop of Mainz with a clear majority over Adolph of Nassau-Wiesbaden-Idstein, although never confirmed by the Pope.

In 1461 he went to Nuremberg for Imperial and Papal reform, and its recommendations earned him the wrath of both the Emperor Frederick III and Pope Pius II.

On the night of 28 October 1462 Adolph captured the city of Mainz, killed 400 citizens and had another 400 including Johannes Gutenberg exiled, and revoked its town charter and status as an Imperial City.

When Adolph died in 1475 he recommended Diether be his successor, and on 9 November that year he was again elected.