Defender was built in 1920 by Haywood of Leigh-on-Sea, Essex. She was a gaff cutter rigged vessel, featuring a short mast, bowsprit, three foresails, and a powerful diesel engine. Traditionally, Leigh cockle boats measured only 36 feet in length, had a shallow draft, and were light enough to be beached as the tide receded. This design allowed crews to collect cockles before the tide returned to refloat their craft.

Defender represented the first of a new, heavier design intended to carry a crew of up to ten, thereby increasing productivity. However, it was soon discovered that her increased weight required her to be beached further offshore, resulting in a longer walk for the crew and less time before the tide refloated the vessel. Only two boats of this new design were ever constructed.

On 31 May 1940, Defender and her sister ships departed for Dunkirk. Defender carried one Royal Naval Volunteer Reserve (RNVR) officer, Sub Lieutenant Soloman—later awarded the Navy's Distinguished Service Cross for valour—who commanded the Leigh cockle boats Letitia, Renown, Endeavour, Reliance, and Resolute.

Between 18:20 and 18:40, the flotilla scattered during an enemy air attack. By 18:50, RAF Spitfires had driven off the attackers, and five Dornier aircraft had crashed into the sea. At 19:15, the reunited flotilla reached Dunkirk Roads but found it impracticable to operate from the beach. Consequently, at 21:30, they began embarking troops from outside the jetty, transferring them to the schuyt Tilly and other vessels.

By 22:40, a strong swell made working from the outside of the mole difficult, so the flotilla entered the harbour in formation and loaded troops for transfer to other ships, including the Sarah Hyde and the Ben & Lucy. At 03:00, the flotilla returned to Ramsgate, with Defender arriving alongside Resolute to disembark soldiers, including the Colonel commanding the Royal Worcestershire Regiment, several officers of the Cameron Highlanders, and 60 men. In total, the Leigh cockle boats rescued approximately 1,000 soldiers during the evacuation.

After the war, Defender was converted into a comfortable sea-going cruising boat, featuring a 50-foot mast and a Bermudan rig. Beneath her shallow draft hull, she was equipped with a drop keel. She was much loved by her owners, including Commander P.F. Clayton RN, his wife, and their six children.

On 22 January 1965, the owners received a letter from the Sunday Times announcing plans for a return by the Little Ships to mark the 25th anniversary of Dunkirk in May. This was the first indication they had that Defender had participated in the evacuation. It took three weeks and thirty-one telephone calls to confirm her involvement and to contact one of her Dunkirk crew members.

Defender returned to Dunkirk in 1965, this time with Ted "Edge" Harvey and George "Pie" Osborne aboard. Osborne had lost his brother Leslie and cousin Frank Osborne when the Renown was sunk on the return journey from Dunkirk in 1940. It was fitting that he was asked to lay a wreath from Defender in memory of all those lost at sea.

Richard Dimbleby provided live television commentary for the return on 6 June 1965, during which Defender gained recognition as a film star. The final shot showed a flag-bedecked Defender slipping out of the Bassin du Commerce with the message "au revoir" spelled out in signal flags at her masthead.

Defender’s involvement with the film industry nearly proved fatal. She was selected as an extra for a television production, and a young director planned to burn her as the climax of a soap opera episode. However, she escaped destruction on that occasion.

Despite her storied history, Defender’s future was uncertain. She fell on hard times and ultimately ceased operation in the autumn of 1999.