In 1675, when his father negotiated with Count Peder Griffenfeld about a marriage with Frederick William's sister, he was promised a profitable post as provost of the Cathedral in Hamburg and governor of Als.

However, when Ernest Augustus reverted to Lutheranism in 1695, he was appointed governor of Sønderborg, because he resided there.

Nevertheless, when Ernest Augustus died childless in 1731, Frederick William's son Christian August I would inherit the duchy.

On 27 November 1694, he married Countess Sophie Amalie of Ahlefeldt-Langeland (1675-1741), the youngest daughter of the Chancellor, Count Frederik of Ahlefeldt-Rixingen, Count of Langeland (1623-1686) by his second wife, Countess Marie Elisabeth of Leiningen-Dagsburg-Hartenburg (1648-1724).

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