During the next few years Monaghan operated primarily in the North Atlantic, training US Navy personnel who served in World War II.

Monaghan opened fire, and at 08:27 was underway to join Ward when notified of the presence of a Ko-hyoteki class midget submarine in the harbor.

Homeward bound, Monaghan, with Dale and Aylwin, made repeated attacks on an enemy submarine, causing it to broach and give off a large oil slick.

With the Japanese threatening Port Moresby, Papua New Guinea, sea lines of communication to Australia and New Zealand were in peril, and the Navy moved quickly and decisively to block so critical a threat.

The opening action of the victory there came 7 May, when American search planes spotted the Japanese occupation force, several transports guarded by the light carrier Shōhō.

Next day, before the major engagement by aircraft from both American and Japanese fleet carriers, Monaghan was ordered away from formation to transmit important messages, thus preserving radio silence in the main body.

Since the position of the sinking had been erroneously reported, Monaghan was unable to carry out a rescue, and sailed on with messages for Nouméa before rejoining TF 16 in time to return to Pearl Harbor 26 May.

The Japanese submarine I-168 penetrated the destroyer screen the next day and sank both Yorktown and Hammann, the carrier remaining afloat another 16 hours before she succumbed.

Once more in the Aleutians, Monaghan joined TG 16.69 a scouting force built around cruisers Richmond and Salt Lake City.

On the night of 21/22 February, she joined in an all-night bombardment on Parry Island, then spent a month on patrol and escort duty in the Marshalls.

On 22 March Monaghan put to sea in the antisubmarine screen for the fast carriers, bound for strikes on Palau, Woleai, and Yap, returning to Majuro 6 April.

While the fliers of TF 58 soundly defeated the Japanese in the Battle of the Philippine Sea, Monaghan's group patrolled off Saipan guarding against a possible breakthrough by the enemy.

There she joined the escort for three fleet oilers bound for a rendezvous 17 December with TF 38, whose planes had been striking central Luzon in support of the Mindoro invasion.

The Typhoon Cobra tragedy, Admiral Chester Nimitz said, "represented a more crippling blow to the Third Fleet than it might be expected to suffer in anything less than a major action".