Leading Seaman Magennis served as Diver in His Majesty's Midget Submarine XE-3 for her attack on 31st July 1945, on a Japanese cruiser of the Atago class.

Owing to the fact that XE-3 was tightly jammed under the target the diver's hatch could not be fully opened, and Magennis had to squeeze himself through the narrow space available. He experienced great difficulty in placing his limpets on the bottom of the cruiser owing both to the foul state of the bottom and to the pronounced slope upon which the limpets would not hold. Before a limpet could be placed therefore Magennis had thoroughly to scrape the area clear of barnacles, and in order to secure the limpets he had to tie them in pairs by a line passing under the cruiser keel.

This was very tiring work for a diver, and he was moreover handicapped by a steady leakage of oxygen which was ascending in bubbles to the surface. A lesser man would have been content to place a few limpets and then to return to the craft. Magennis, however, persisted until he had placed his full outfit before returning to the craft in an exhausted condition.

Shortly after withdrawing Lieutenant Fraser endeavoured to jettison his limpet carriers, but one of these would not release itself and fall clear of the craft. Despite his exhaustion, his oxygen leak and the fact that there was every probability of his being sighted, Magennis at once volunteered to leave the craft and free the carrier rather than allow a less experienced diver to undertake the job. After seven minutes of nerve-racking work he succeeded in releasing the carrier.

Magennis displayed very great courage and devotion to duty and complete disregard for his own safety.

( Leading Seaman James Magennis's other crew member in the midget submarine was Lieutenant Ian Fraser, Royal Naval Reserve, who was also awarded the Victoria Cross ).

James Magennis and Ian Fraser were invested with their Victoria Crosses by King George VI at Buckingham Palace on the 11th December 1945.