Carmania entered service for Cunard Line in 1905, having been christened by Lady Blythswood, the wife of a former aide to Queen Victoria. She and her sister Caronia were the largest ships in the Cunard fleet at the time of their maiden voyages and were designed to be near identical in every aspect except for their machienary.
The two ship's were used by the line to examine merits of two competing propulsion units. Carmania was fitted with the revolutionary yet relatively untested steam turbine engines, while Caronia was completed with traditional quadruple-expansion engines.
After a relatively short time it became apparent that Carmania, with her steam turbines, was both faster and more efficient that Caronia. This important test case convinced Cunard to opt for turbine propulsion in their new super liners; Mauretania and Lusitania.
Carmania ran on the Cunard New York service. In June 1910 a fire broke out aboard while alongside in Liverpool. Although the damage was extensive, it was contained to the passenger accommodation, leaving the valuable machinery undamaged. The ship was subsequently repaired and returned to service.
In October 1913 Carmania received an SOS from the Canadian Northern Steamship Co., liner Volturno. Despite reaching the scene in four hours the weather was so bad that Carmania was forced to stand by until the storm weakened, resulting in the loss of 103 passengers and 30 crew.
Following the outbreak of World War I, Carmania was called into millitary service. The ship was transformed into an Armed Merchant Cruiser and entered wartime service on 23 August 1914.
Carmania was involved in a fierce battle with the former Hamburg-Amerika liner Cap Trafalgar on 14 September 1914, sinking her advisory. However Carmania was herself extensively damaged and required refurbishment in the dry dock at Gibraltar.
For the remainder of the war Carmania patrolled seas off the coast of Portugal, and was also involved in the disastrous Gallipoli campaign.
Following the armistice, Carmania was returned to Cunard and refurbished. She was used to repatriate Canadian troops before returning to regular passenger services in 1920. She returned to the Liverpool to New York service and during 1923 the passenger accommodation was altered to allow for 425 cabin class, 365 tourist class and 650 3rd class passengers.
She continued in this role until 1931 when she was withdrawn from service and subsequently scrapped.