Leopard began life as SS Yarrowdale, a British cargo steamer of 4,652 gross register tons, operated by Robert Mackill & Co of Glasgow. On 11 December 1916, the German commerce raider Mowe captured her in the Atlantic. Mowe's commander, Nikolaus zu Dohna-Schlodien, recognized Yarrowdale's potential: the right size, decent speed, and, crucially, an unremarkable appearance. He loaded 469 prisoners aboard, including 89 American citizens, and sent her to Germany under a prize crew. A strike by Liverpool boilermakers had thinned the British Northern Patrol from 23 ships to six, and Yarrowdale slipped through on Christmas Eve 1916. By 9 January 1917, the Kaiserliche Werft Kiel had renamed her SMS Leopard and armed her with nine guns hidden behind false doors and shutters, plus two torpedo tubes. She could outgun any ship smaller than a modern cruiser.

Disguised as the Norwegian freighter Rena Norge, Leopard sailed on 7 March 1917 under Korvettenkapitan Hans von Laffert, tasked with relieving Mowe on the Atlantic shipping lanes. The genuine Rena was off South Africa at the time. German intelligence had been reading intercepted British wireless messages and provided Laffert with detailed information on the Northern Patrol's dispositions. But neither side had fully grasped a dangerous symmetry: if you could decipher your enemy's signals, your enemy could likely decipher yours. On 10 March, Laffert received word that the British had changed their cipher, cutting off German intelligence. He was advised to postpone. His reply was terse: "Have received telegram, long live the Emperor." He pressed north.

The British armoured cruiser HMS Achilles and boarding steamer Dundee were on patrol north of the Shetlands on 16 March, the day appointed for their relief. Near noon, a ship appeared to the east. Achilles chased for hours before overhauling her and ordering Dundee to send a boarding party. Day's suspicions mounted as he watched the ship's continuous turning, a maneuver that would bring her hidden guns broadside-on to Dundee. He kept his vessel at right angles, gun crews at the ready. Then the painted Norwegian flag on Leopard's hull fell off into the water, and the game was over.

Day gave the order to fire. At point-blank range, every shell from Dundee's 4-inch guns found its mark, raking Leopard's stern while the 3-pounder hammered the bridge. Twenty hits on the bridge may have killed the command crew early. Two torpedoes streaked past Dundee's stern, missing by a narrow margin. Day anticipated Laffert's starboard turn and swung to port, denying Leopard a broadside. By the time Achilles opened fire five minutes later, Leopard had already taken forty hits and was trailing smoke. Dundee exhausted her ammunition and sheltered behind Achilles, which continued pounding the raider for another hour. Internal fires triggered explosions that sent columns of flame through black smoke. One of Leopard's guns kept firing almost to the end. Shortly after, Leopard listed to port and sank. All 319 German crew and the six British boarders died. No search for survivors was made, for fear of submarine attack. The rowing boat that had carried Lawson's party was recovered by a merchant ship months later, empty.

Leopard was the final commerce raider Germany sent to sea during the First World War. After her loss, only days into her first patrol, the Imperial Navy abandoned surface raiding entirely and relied on unrestricted submarine warfare, which had resumed on 1 February 1917. A bottle found after the action contained a message from a crewman: "In action with British cruiser. Fighting for the glory and honour of Germany. A last greeting to our relatives." The spot where Leopard sank, roughly 64.9 degrees north and 0.37 degrees east, marks the grave of 325 men on both sides of a war that had made the northern seas as lethal as any trench on the Western Front.