He was filled with a burning desire for adventure, and, at the close of 1674, when in his seventeenth year, he embarked in Admiral Torrington's ship, and proceeded to join as a volunteer Sir John Narborough's fleet in the Mediterranean, in order to take part in the expedition to restrain and revenge the piratical depredations of the barbarous states of Tripoli and Algiers.
He distinguished himself on the 14th of January, 1675, in an attack by the boats of the fleet upon four corsair men o' war moored under the very guns of the castle and fort of Tripoli.

The exploit was a successful one, the ships were all burned, and most of their crews slain.

Another encounter with the fleet of Tripoli took place in February, when the pirates were again defeated, and the bey forced to grant all the English demands.
In 1677 the fleet returned to England, and with it Mordaunt, who had during his absence succeeded to his father's title and estates, John Lord Mordaunt having died on the 5th of June, 1675.

Shortly after his return to England Lord Mordaunt, though still but twenty years old, married a daughter of Sir Alexander Fraser.

But his spirit was altogether unsuited to the quiet enjoyment of domestic life, and at the end of September, 1678, he went out as a volunteer in his majesty's ship Bristol, which was on the point of sailing for the Mediterranean to take part in an expedition fitting out for the relief of Tangier, then besieged by the Moors.