On 10 April 1653, he was made commandeur to replace flag captain Abel Roelants when Lieutenant-Admiral Maarten Tromp used Brederode as his flagship.

Kortenaer kept Tromp's standard raised to keep up morale (this was habitual for the Dutch on such occasions) and took command of his squadron.

In the Battle of the Sound (8 November 1658), serving as flag captain on Eendragt, he beat off every Swedish attack while his commanding officer, Lieutenant-Admiral Jacob van Wassenaer Obdam, was debilitated by gout.

After this heroic conduct against the Swedish, Kortenaer was promoted to vice-admiral on 8 May 1659 and knighted by Frederick III of Denmark in the Order of the Elephant.

He was fatally wounded early in the battle on Groot Hollandia by a cannonball hitting his hip and buried in Rotterdam in a marble grave memorial engraved with a poem by Gerard Brandt: Several ships of the Dutch navy have been named Kortenaer after the admiral,