He succeeded his father as Duke of Saxe-Hildburghausen when he was only eighteen years old in 1745; as a result his mother, the Dowager Duchess Caroline, acted as a regent on his behalf until he reached adulthood, in 1748.
Ernst Frederick was considered to be intelligent, talented, and one of the most beautiful princes of his time. He donated a library to the city, but finally his excessive prodigality in exaggerated yard and military splendor drew the attention of the highest places to the financial situation of his country.
The Emperor Joseph II created a Debit Commission under management of the Duchess Charlotte Amalie of Saxe-Meiningen and prince Joseph of Saxe-Hildburghausen, the grand-uncle of the duke, to investigate the demands of the creditors and adjust the incomes and expenditures to 1769. The financial situation of the country was so disastrous that 35 years duration of this commission could not arrange conditions completely.
After he made use in 1757 of the Münzregal (Imperial Regalia), he was entangled in a complaint of the realm treasury. Finally, the huge fire of the city of Hildburghausen in 1779, forced Ernst Fredercik to move to his hunting residence in Seidingstadt, where he died a year later.
In Bayreuth on 1 July1758, nine months after the death of his second wife, Ernst Frederick was married for the third time to Ernestine of Saxe-Weimar. They had three children: