He was the seventh child but fourth surviving son of Wilhelm, Duke of Saxe-Weimar and Eleonore Dorothea of Anhalt-Dessau.

Subsequently, he was pulled into political affairs when his father sent him to Paris in order to strengthen the relations of Ernestine line with the King Louis XIV, hopefully through a marriage.

They had one daughter: Meanwhile, Bernhard's efforts to have his marriage annulled were unsuccessful, as no theologian or jurist could give him grounds for divorce; and he appeared to reconcile himself with Marie Charlotte.

Nevertheless, on 20 October 1672 he promised in writing to his mistress that he would never forget her, but would care and protect her as if she were his true wife, and giving her the style of "Lady of Alstädt" and an annual rent of 1000 Taler.

When Bernhard died, at the age of 39, was succeeded by his only surviving son Johann Wilhelm, born of his first wife after their reconciliation in 1675.