He served in the Royal Canadian Navy Volunteer Reserve during the Second World War and reached the rank of Lieutenant Commander. He was the youngest Commander of a Flower Class Corvette ship.

Lt Gardiner commanded three ships during the war. The first was the Flower class Corvette, HMCS Atholl from Oct 1943 to Sept 1944. He then briefly commanded the Frigate, HMCS Eastview in Sept 1944, and then returned to command another Corvette, the HMCS St. Lambert from Oct 1944 until June 1945.

The Corvettes he commanded were involved in convoy escort and anti-submarine warfare and in all he made 38 successful crossings of the Atlantic Ocean during the war.