When launched, Carmania and her running mate,, were the largest ships in the Cunard fleet. Carmania had steam turbines and Caronia had quadruple-expansion engines. The essentially identical ships with the two different engine suites was seen by the industry as an opportunity to compare operations and removed all doubt about the advantages of turbine engines. Another feature that differentiated the two liners was that Carmania had two tall forward deck ventilator cowls, which were absent on Caronia. Carmania left Liverpool 2 December 1905 for her maiden voyage to New York arriving 10 December making the voyage in 7 days, 9 hours and 31 minutes for 15.97 knots over the 2,835 mile route. The ship traveled the New York-Liverpool route from 1905 to 1910. In the spring of 1906, she carried H.G. Wells to North America for the first time; he noted in a book about his travels, "This Carmania isn't the largest ship nor the finest, nor is to be the last. Greater ships are to follow and greater". Carmania suffered a major fire in June 1910. In October 1913, while eastward bound, she responded to a distress call from to pick up survivors in a storm, which resulted in many awards for gallantry being presented to various members of her crew and Captain James Clayton Barr. Following the outbreak of World War I, Carmania was converted into an armed merchant cruiser, equipped with eight 4.7-inch guns, and put under the command of Captain Noel Grant. She sailed from Liverpool to Shell Bay in Bermuda. She subsequently engaged and sank the German merchant cruiser, during the Battle of Trindade. At the time Cap Trafalgars appearance had been altered to resemble Carmania. The ship suffered extensive damage herself and several casualties to her crew. After repairs in Gibraltar, she patrolled the coast of Portugal and the Atlantic islands for the next two years. In 1916, she was summoned to assist in the Gallipoli campaign. From March 1916, she was used as a troop ship. After the war, she transported Canadian troops back from Europe. By 1919, she returned to passenger liner service and was refitted in 1923. In 1932, she was sold to Hughes Bolckow & Co., and scrapped at Blyth. The Carmania's bell is on display aboard the permanently moored HQS Wellington at Embankment, London, UK.
