Mulberry Harbours were, in essence, pre-fabricated harbours. Constructed in the UK, their components could be towed over the English Channel and then be used to turn an empty beach into a port for offloading men, vehicles and supplies. Two such harbours were constructed; one in the Anglo-Canadian zone at Arromanches (on Juno Beach), and one in the American zone at Omaha Beach. There were also smaller, simpler 'Gooseberry' harbours, with one at each beach; the Gooseberries for Juno and Omaha would form part of the Mulberries for their respective beaches.

A Mulberry Harbour was composed of four main components; two to protect the ships from the action of the sea and two to aid offloading. The first component a ship entering a Mulberry would encounter were the 'Bombardons', giant steel structures forming an outer breakwater. This was then supplemented by the 'Phoenixes'. These were hollow concrete structures, five stories high, which were flooded and sunk in place. Once the ship was inside the sheltered water created by the Bombardons and Phoenixes, they would approach the 'Spud' pierheads, floating piers onto which ships could unload. These were then connected to the shore by the 'Whale' floating roadways, which floated on 'Beetle' pontoons. There were also 'Dolphins', extendable tubular structures used to secure the sterns of Landing Ships Tank (LSTs), which unloaded over the bow, rather than alongside. The Gooseberries were much simpler, being composed of 'Corncobs' - old merchant ships and obsolete warships which were scuttled to form breakwaters. The Mulberry harbours had four 'Whale' piers, protected by 11-15 Corncobs, ~60 Phoenixes (forming two lines on either flank of the harbour), and several miles of Bombardons. Each had an area of roughly two square miles, comparable in size to the port of Gibraltar - enough space for seven Liberty ships and twelve smaller vessels.

The genesis of the Mulberries came following Britain's withdrawal from Dunkirk. It was clear that Britain would need to invade Europe to defeat Germany. To do so would require a significant logistical capacity, which could only be provided by capturing a port; but this was clear to the Germans and so the ports would be very difficult to capture. Indeed, the Dieppe Raid of 1942 proved this to be correct. A number of British officers and engineers suggested, in 1940-41, getting round this by constructing artificial harbours. These only really found traction after May 1942, when Churchill demanded piers that 'float up and down with the tide'. Initial tests of various designs of floating roadways and pontoons began on the West Coast of Scotland in late 1942-43. In August 1943, the project was approved by a joint Anglo-American Committee, and construction began soon after. Construction was somewhat of a bureaucratic nightmare. The Admiralty was responsible for the overall design and operation of the harbours, as well as the construction of the Bombardons and Gooseberries, while the Ministry of Supply was responsible for all other components of the Mulberries. Sites for the harbours were chosen in the run-up to D-Day, using a combination of aerial photographs of the beaches, old charts, and surveys made by special forces teams.

The various components of the Mulberries were assembled on the South coast of England. Because the towed ships and caissons were so slow, they had to set off several days before the rest of the invasion fleet. The first components arrived off the beaches on the 7th, with three Corncobs being scuttled off Omaha. The next day, the first Phoenixes were put into place. By the 11th, all five Gooseberries had been fully constructed, while work continued on the Mulberries. The first vehicles arrived at the Omaha Mulberry at 16:30 on the 16th June. Thirty eight vehicles were offloaded from an LST directly onto the operational pontoon. Eleven LCTs (Landing Craft Tank) would follow over the remainder of the 16th and on the 17th. Unloading onto a Mulberry was considerably quicker than onto the beach; it took just over an hour to unload an LCT onto a Mulberry, while it would take 10-12 hours to unload over the beach. The Mulberry at Arromanches took longer to fully come into operation, but the breakwaters were more complete than at Omaha. This was significant because the beachhead was struck by a strong storm, starting on the night of the 18th-19th. The Omaha Mulberry, with fewer Bombardons in place, suffered greatly from the storm; the Phoenix caissons had also been left uncovered, allowing waves to break over and inside them, a stress they'd not been designed to protect against. Many failed, allowing the waves to wreck the harbour. The Arromanches harbour, with more protection from the more established breakwaters, as well as from the Calvados Reef and from the shallower water it was established in, would survive, albeit with heavy damage to its Bombardons. The Americans would abandon the Omaha Mulberry as a result of the storm damage, choosing to unload over the beaches instead. The British cannibalised the remains of the harbour to bolster the Arromanches harbour, bringing it into full operation on the 29th June. It reached the planned 6,000 tons of supplies landed per day on the 8th July, and, following the establishment of a dedicated floating pier for LSTs, surpassed 7,000 tons on the 19th. A record of 11,000 tons of supplies unloaded was set on the 29th July. While the British Mulberry did meet expectations, it's hard to say how useful it was, as the Americans did manage to unload more supplies over Utah and Omaha beaches without the Mulberries.