After steaming to the U.S. East Coast, Mainstay underwent shakedown in Chesapeake Bay and joined Mine Division 32 on 21 July.

She closed the French coast 20 August, and during the next two months she swept for enemy mines from Marseilles, France, to San Remo, Italy.

While sweeping coastal waters between Menton, France, and Ventimiglia, Italy, she came under fire from German shore batteries on 9 and 10 September and engaged the enemy guns with 3-inch gunfire.

Mainstay patrolled the Provence coast until 25 October thence, after overhaul at Bizerte, Tunisia, she swept for enemy mines off Cagliari, Sardinia, and Palermo, Sicily.

Departing 1 September, she steamed via Eniwetok and Saipan to Okinawa; and, after arriving 1 October, she put to sea the 7th to avoid an impending typhoon.

A week later she departed for the eastern coast of Korea, and on 2 January 1951 she began coastal patrols and mine sweeps in support of the Blockading and Escort Force.

As American and U.N. ground forces began their second drive up the peninsula, she swept for mines and fired at enemy shore positions during an amphibious feint on 30 and 31 January against the Communist-held Kansong-Kosong area.

She ranged the coastal waters from Pohang in the south to Songjin in the north, and between 21 February and 31 May she operated primarily off Wonsan supporting the siege and bombardment of the harbor.