He was the fourth son of Joachim Ernest, the reigning duke of Holstein-Plön, and his wife Dorothea Augusta of Gottorp.
Joachim Ernest had his son trained militarily and Bernhard became a colonel in the Spanish service at first.
In August 1675 he returned to Plön, where he took command of the troops there.
Denmark was preparing the Pomeranian campaign of 1675, and on 25 October 1675, he was appointed Major General in the Danish army.
He was succeeded as commander of the Danish troops by his older brother John Adolphus.