The Carmania and her sister ship, the Caronia, entered service for Cunard in 1905 and at the time they were the largest ships in the Cunard fleet.
Following an examination of the merits of different propulsion units, the Carmania, was fitted with steam turbines while the Caronia, was fitted with quadruple-expansion engines. This offered a test case for Cunard to trial each engine type and compare their merits for future ships.
The Carmania was christened by Lady Blythswood, the wife of a former aide to Queen Victoria. The fact that the Carmania was reported to be the forerunner of a larger turbine ship, (later to become known as the Lusitania), was of great public interest. Carmania made her maiden voyage from Liverpool to New York, on 2nd December 1905. Several engineering experts had been on board the Carmania during the maiden voyage, and were enthusiastic about how the engines had performed smoothly and efficiently. The ship was powered by three direct acting Parsons steam turbines.
The Carmania continued to operate the New York service without major incident for some years. In June 1910, whilst the ship was in dock at Liverpool, a fire broke out which took the fire brigade some time to extinguish. Although the damage was quite considerable, it was only to the passenger accommodation, and the structure and machinery were undamaged.
In October 1913, whilst traveling from New York to Liverpool, she received an SOS call from the emigrant ship Volturno, of the Canadian Northern Steamship Co.Ltd. The Volturno had been traveling from Rotterdam to America with emigrants and a cargo of barium oxide, which had broken loose in gales and caused a fire. The Carmania, under Captain Barr, reached the scene 4 hours after receiving the signal. The ship was forced to stand-by all night as the weather was so severe. The next day the Carmania, and several smaller vessels that had come, picked up the survivors. 103 passengers and 30 crew from the Volturno were lost. Several awards for gallantry were made to the crew of the Carmania.
After the outbreak of World War I the Carmania was requisitioned by the government and converted into an Armed Merchant Cruiser. Under the command of Captain Noel Grant RN, the ship sailed from Liverpool and arrived at Shell Bay in Bermuda on 23rd August 1914. A German ship, formerly of the Hamburg-America Line, the Cap Trafalgar, had been reported in the vicinity but was believed to be headed for South Africa. The ship, however, had been armed and was patrolling around Trinidad. On the morning of 14th September the Carmania engaged the Cap Trafalgar. Despite having to abandon the bridge of the the ship, due to fire, the crew of the Carmania continued to fire on the German ship. The Cap Trafalgar began to list and went down bows first. The battle had lasted nearly one hour. The Carmania was potted with holes from Cap Trafalgar's bombardment, and had lost 7 men in the action. The ship was then escorted to Gibraltar and placed in dry dock.
By 23rd November repairs were complete. Until May 1915 the ship patrolled the coast of Portugal and the Atlantic Islands, but was afterwards required to assist in the Gallipoli campaign. It also assisted in quelling a mutiny on board the British steamship Maristan. In May 1916 the Carmania was allowed to return to Cunard and, after being refitted, was employed largely on trooping duties between Halifax and Liverpool. After the end of the war it was engaged in the repatriation of Canadian troops.
At the beginning of 1920 she was completely reconditioned and returned to the Liverpool to New York service. In 1923 the passenger accommodation was altered to allow for 425 cabin class, 365 tourist class and 650 3rd class passengers. Despite being involved in several minor collisions the rest of the Carmania's career was relatively uneventful. Like her sister ship she was employed on short cruises in the winter months. By 1931 the ship had become outdated and overshadowed by more modern vessels, and in March 1932 the Carmania was sold to Hughes Bocklow & Co. and scrapped at Blyth.