Ernestine Auguste Sophie was a daughter of the Duke Ernest August I of Saxe-Weimar-Eisenach and his second wife, Margravine Sophie Charlotte of Brandenburg-Bayreuth, daughter of George Frederick Charles, Margrave of Brandenburg-Bayreuth.

Ernest Frederick Charles was heavily indebted and the dowry Ernestine brought in was significant.

Carl Barth describes the Duchess as follows: "... apart from a fine half-squint of one eye, she was a beautiful, well-built lady who occupied herself passionately with music (French horn, flute, piano and violin!).

His policies improved the country's tight financial situation, but he could not prevent a national bankruptcy.

In 1769 the country was placed under imperial sequestration and a debit commission attempted to consolidate the financial situation.