During the summer of 1940, after shakedown and an extended training period, Mayrant escorted her Commander in Chief, Franklin D. Roosevelt, on a tour of east coast defenses.

By September, it was officially responsible for protecting them as far as Iceland, lengthening the patrols of the Support Force, Atlantic Fleet, which had been assigned the task.

In August she stood-by during the Atlantic Charter Conferences and, at their conclusion, escorted HMS Prince of Wales, carrying Prime Minister Winston Churchill, to Great Britain.

As a unit of that fleet she participated in operations in the Denmark Strait in search of the German battleship Tirpitz in addition to escorting several convoys on the "suicide run" to Murmansk.

Mayrant returned to the east coast in July and immediately put her experience to work conducting antisubmarine warfare training exercises in the Caribbean.

However, heroic action of her crew, and able assistance of several other ships who pumped water and provided electric power, kept the Mayrant floating as she crawled back to harbor.

For the next year she operated primarily along the east coast, escorting new cruisers and aircraft carriers on shakedown and protecting coastal convoys.

On patrol off New England, 5 April 1945, Mayrant went to the rescue of the cargo ship Atlantic States, torpedoed off Cape Cod Light.

For 2 days until oceangoing tugs had her under control, they battled waves and breaking lines to keep Atlantic States from drifting and sinking.

After the end of hostilities, Mayrant was designated to make preliminary arrangements for the surrender of the enemy garrison on Marcus, a bypassed island in the central Pacific.