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.

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.

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, the third surviving child and second daughter of Charles I, Duke of Brunswick-Wolfenbüttel and his wife, Princess Philippine Charlotte of Prussia, sister of Frederick the Great.