On 2 January 1672 John Hugh's uncle Archbishop Charles Casper of the Leyen appointed him as coadjutor of Trier and his successor. Charles Casper died in 1676, and John Hugh became the archbishop on the 1st of June. Upon taking the archbishopric throne, John Hugh demanded a sum of 150,000 Thalers, however due to the poverty of the country only 86,000 Thalers were collected. John Hugh became immediately concerned with the annexations and expansion of France under King Louis XIV, which pressed on Trier. John Hugh appealed to the Holy Roman Emperor on 22 June 1680 for assistance against France, but Leopold I had adopted a policy of cordiality towards France and as such was not willing to assist.