Appointed Rear Admiral 20 October 1910, he departed the Philippines on 3 February 1912 to become Governor of the Naval Home, Philadelphia, Pennsylvania on 25 March.

After shakedown off Bermuda, Mertz departed Norfolk, Virginia, 26 January 1944 for the central Pacific, via the Panama Canal and San Diego, California, arriving at Pearl Harbor 5 March.

Five days later Mertz steamed for the New Hebrides as a screen for the escort carrier Barnes, arriving at Espiritu Santo on 15 April.

Early in the morning of 25 October as the Japanese Southern Force approached Leyte Gulf through the Mindanao Sea, Mertz and McNair patrolled between Desolation Point and Homonhon Island, lest the enemy fleet choose to steam north along the east coast of Dinagat Island to attack the Allied beachhead.

With the Leyte beachhead established, the destroyer got underway 26 October for Hollandia, New Guinea, anchoring in Humboldt Bay on the 30th to replenish.

Mertz then continued on to Seeadler Harbor, Manus, to join a task unit staging for the daring expedition through the Sulu Sea, controlled by the enemy since early 1942, to capture Mindoro.

As part of Adm. Willam F. Halsey’s 3rd Fleet she cleared San Pedro on 1 July, and nine days later arrived at the launching area off the southeast coast of Tokyo for strike on the Japanese home islands.

Beginning with the attacks on Tokyo 10 July, Mertz ranged up and down the coasts of Japan until she joined an antishipping sweep in the Kuriles while en route to the Aleutians.

On 1 December, Mertz steamed to San Diego, where she decommissioned 23 April 1946 and entered the Pacific Reserve Fleet.