Ernst August II Konstantin's father, a splendor-loving ruler with a passion for hunting, had moved his court to Eisenach. The duke neglected his son and heir, so that Ernst August II Konstantin spent his early years under the supervision of the Hofmarschall of Schloss Belvedere in Weimar.

Ernst August I died in 1748, when Ernst August II Konstantin was eleven years old. Since he was still a minor, the dukes Frederick III of Saxe-Gotha-Altenburg and Franz Josias of Saxe-Coburg-Saalfeld assumed the regency of Saxe-Weimar-Eisenach on Ernst August II Konstantin's behalf. The young duke came to live with Duke Frederick in Gotha, who made sure that Ernst August II Konstantin received an appropriate education.

In 1755 Ernst August II Konstantin assumed the reins of government. He appointed his former tutor, the Imperial Count (Reichsgräf) Heinrich von Bünau, as his new chancellor. Because the young duke had been a sickly child, he was encouraged to marry quickly in order to ensure an heir for the duchy.

In Brunswick on 16 March 1756, Ernst August II Konstantin married Anna Amalia of Brunswick-Wolfenbüttel. They had two sons:

When Ernst August II Konstantin died, the hereditary prince Karl August was still an infant. Ernst August Konstantin's widow, the duchess Anna Amalia, presided as regent over an excellent tutelary government which propelled Weimar into the classical period.

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