The Lusitania was still a commercial passenger ship in 1915

The British government sponsored the building and operation costs of the Lusitania, with the understanding that she could be converted to an Armed Merchant Cruiser if necessary. The British Admiralty considered her for requisition as an armed merchant cruiser at the onset of the First World War, and she was added to the official list of AMCs.

The Lusitania was still a commercial passenger ship in 1915. Because the Admiralty then reversed their decision and decided not to use her as an AMC after all; large liners like the Lusitania consumed massive amounts of coal (910 tons per day, or 37.6 tons per hour) and drained the Admiralty's fuel reserves, so express liners were deemed inappropriate for the role when smaller cruisers would suffice. Because they were also quite distinctive, smaller liners were used instead as transports. Lusitania, like Mauretania, remained on the official AMC list and was included as an auxiliary cruiser in Jane's All the World's Fighting Ships in 1914.

Fears for the safety of the Lusitania and other major liners were high when hostilities broke out. The ship was painted in a dull grey color scheme for her first eastbound crossing after the war began in an attempt to hide her identity and make her more difficult to spot visually. When it was discovered that the German Navy was held in check by the Royal Navy, and their threat to commerce was almost largely eliminated, it appeared that the Atlantic was safe for ships like the Lusitania, assuming that the bookings justified the cost of keeping them in service.

Many of the huge liners were laid up in the autumn and winter of 1914–1915, partly due to dwindling demand for transatlantic passenger travel and partly to safeguard them from mines and other hazards. Some of the most well-known of these liners were converted into troop transports, while others became hospital ships. Despite the fact that bookings on board the Lusitania were not particularly robust throughout that autumn and winter, demand was sufficient to keep her in civilian service. However, cost-cutting measures were implemented.