She served primarily in the Europe–Africa–Middle East Theater from August 1943 until January 1946, but also transported British forces from Calcutta, India, and landed them as Regu Beach, Burma.

This group of LSTs carried landing craft tanks (LCTs) on deck for delivery at destination.

At 19:00 that evening, LST-21 set sail as flagship, under Commander W. S. Blair, for Syracuse, Sicily; in company with six LSTs.

On 29 September, they took aboard 27 tanks and 18 vehicles as well as 184 officers and men of the 5th and 12th Canadian Transport Regiments.

They heaved to off the harbor and were boarded by British Naval Officers in Charge, who instructed them to proceed to Manfredonia to discharge the cargo.

[9][2] On 2 October, they departed Manfredonia, proceeded south along the Italian Coast and anchored at 18:00 in Brindisi Harbor.

At 15:40 on 3 October, they departed Brindisi Harbor as additional escort for SS Ocean Vesper and proceeded toward Taranto.

[10][2] LST-21 sailed from Algiers, Algeria, to Port Said, Egypt, in October 1943, this time joining with Convoy UGS 19.

Embarking officers and men of the 15th Indian Corps as well as 13 "Lee" medium tanks, she was underway on 3 December, rendezvousing with LST-25 on 4 December, under escort of two Royal Indian Navy launches, with one B-24 Liberator and four Spitfires as aerial escorts.

[13] LST-21 joined Convoy MKS 38 at Bizerta, Tunisia, in January 1944, as it was en route to Gibraltar, arriving 1 February.

They sailed without incident until 5 February, when, at 09:30, one of the destroyer escorts let go with a white flare and started screening the starboard side of the convoy with a smokescreen.

[18] On 7 February 1944, at 08:25, an escort off of the starboard beam hoisted the black pennant and dropped a depth charge.

At 19:30, fog set in and all ships were out of sight of LST-21 which turned on navigation lights and sounded whistle to avoid collision.

The escorts and planes were noted to be having a difficult time with communications due to all of the contacts, real or imagined.

[20] On 10 February 1944, soon after daybreak, the Convoy Commodore sent out another message to all ships announcing that a fourth submarine had been sunk during the night.

On 11 February, LST-21 and the other LSTs of Commander William Blair's group (except LST-72 and LST-261 which remained in Oran for repairs) were detached off the southwest coast of Ireland and given one escort.

On 1 April, they reached Lough Foyle, and anchored off Moville, before proceeding toward Derry with a pilot, and to the naval base at Lisahally.

On 16 April 1944, she was transferred with other ships of LST Division 101, Group 51, Flotilla 17, from detached duty with the 11th Amphibious Force to British operational control.

At 12:10 on 6 June 1944, LST-21 arrived in the "Gold Beach" assault area and cast off the Rhino ferry.

British cruisers lying about two miles (three point two kilometres) off shore carried out a naval bombardment of that area.

At 11:46 shells from another German 8.8 cm gun began falling near ships in the area and a British destroyer north of LST-21 engaged the shore battery.

The LST took on 13 casualties from a DUKW and the Rhino ferry returned at 21:45 and departed at 22:40 with the remaining vehicles disembarked from LST-21.

There were intermittent alerts and anti-aircraft fire during the morning of 7 June 1944, and in the dive-bombing attack that followed, HMS Bulolo, 1,000 yd (910 m) north of LST-21, received a bomb hit on the forecastle.

[2] Mooring at Southampton on 8 June 1944, LST-21 discharged casualties and loaded 40 vehicles and 146 Army personnel.

Made smoke on red alert and observed considerable anti-aircraft fire and bomb bursts.

Enemy aircraft were active intermittently during the early morning of 11 June 1944, and at 09:35 on that date she joined a northbound convoy for the Thames River.

[2] Arriving on 12 June, she proceeded to the King George Fifth Docks in London and moored to take aboard 31 English ammunition trucks and 131 Army personnel.

Unloading was completed at 00:37 on 15 June 1944, and the LST retracted from the beach and proceeded to the outbound sailing anchorage.

She proceeded alone at best speed and arrived off Calshot, England, reporting damage and remained anchored from 16 to 18 June, awaiting availability at Southampton Repair Docks.

The ship arrived on Thursday, 25 October, coming from Casco Bay, Maine, with a crew of 100.