The first plates of her keel were laid in 1910 at the Vulcan Shipyards in Hamburg, and she made her maiden voyage in 1913. At 51,680 gross tons, the Imperator was the largest ship in the world until the Vaterland sailed in 1914.
Before her launch on 23 May 1912, in order to make her longer than the , which was under construction at the time at the John Brown shipyards in Glasgow, she was fitted with a large bronze eagle created by Professor Bruno Kruse of Berlin which graced her forepeak with a banner emblazoned with HAPAG's motto Mein Feld ist die Welt (My field is the world). The eagle's wings were later torn off in an Atlantic storm during the 1914 season, when it was removed and replace with gold scrollwork which was similar to what was on her stern.
On her intial sea trials the ship ran agound (similar to the Queen Mary's first journey) on the Elbe river due to insufficent dredging and a flash fire in the engine room which resulted in eight crewmen being taken to hospital, on her official trials she sufferred overheating of the turbines and some stability issues were discovered so, the trial were abandoned and the builders were called in for emergency work, at this time the overnight cruise for the Kaiser was cancelled and eventually carried out in July that year.
Imperator left on her maiden voyage on Tuesday 10th June 1913 with Commodore Hans Ruser in command With Hamburg-Amerika appointing four other captains for the journey to make sure that everything went smoothly, on the way she stopped at Southampton and Cherbourg before making her way accross the Atlantic to New York. On her first arrival the harbour pilot assigned to bring her into the Ambrose channel, Captain George Seeth noted that the ship listed from side to side when the helm made changes to the ships direction and she was soon nicknames "Limperator".
The Imperator returned to the Vulkan shipyard in October 1913 to carry out drastic stabilty work to improve the handling as his had been discovered that the centre of gravity for the ship was too high. In order to correct the problem and lower the ships centre of gravity all the marble bathroom suites in first class were removed and all of the heavy furniture was replaced with lightweight wicker cane and finally 2000 tons of cement was poured into the ships double bottom to cure the vessels seaworthiness issues, this work cost £200,000 which had to be bourne by the shipyard as part of their five year warranty to the shipowners, at the same time an advanced fire sprinkler system was fitted throughout the ship after serveral fires had occured on board since the vessel had entered service.
During the 1914 refit of the Imperator Commodore Ruser handed over command of the ship to Captain Theo Kier and left to take command of the new larger flagship Vaterland which was nearing completion. The Imperator returned to service on 11th March arriving at New York five days later on the 19th.
Among her luxurious features, the Imperator introduced a two-deck-high, Pompeiian-style swimming pool for her first-class passengers.
She was commissioned as USS Imperator (ID-4080) in early May 1919 under captain Casey. After embarking 2,100 American troops and 1,100 passengers, Imperator departed Brest on 15 May 1919, arriving at New York City one week later. Operating with the Cruiser and Transport Force from 3 June to 10 August, she made three cruises from New York to Brest, returning over 25,000 troops, nurses, and civilians to the United States.
While en route to New York City 17 June, Imperator assisted the French cruiser Jeanne d'Arc, which had broken down in the Atlantic Ocean. The President of Brazil was on board Jeanne d'Arc and Imperator received him and his party for transport to the United States, arriving there several days later.
Decommissioned at Hoboken in early 1919, Imperator was transferred to the British shipping controller on 20th September and it was decided that she would operated by Cunard. Captain Charles A Smith and a full crew was sent out to New York on the Carmania by her new operators and the official handover from the American board of shipping to Cunard took place on on November 24th where the vessel was offically received by Cunards Marine superintendant Captain Miller accompanied by his assistant Captain Palfrey. Imperator was then transferred to Cunards pier 54 for Cunard service.
The ship arrived at Southampton on Sunday 10th December 1919 and then to Liverpool for a quick overhaul as she was scheduled to leave on her first voyage for her new owners on January 10th 1920, the ship was found to be in a poor condition, during the drydocking on January 6th it was found that the rudder had a peice missing and that the propellers were suffering from erosion on their leading edges, all of this would have be repaired before service could commence. The ship was then furnished with items brought over from other Cunard vessels such as Transylvania and Carmania.
Due to the extent of the work that had to be carried out the Imperator stayed at Liverpool until the 21st February and during this time the companies annual dinner was held on board before the ship returned to service on the North Atlantic.
On the 2nd March 1920 the ship left New York and took nine days to reach Southampton and during the voyage Imperator developed a severe list which was found to be a faulty ash ejector, at this point Cunard decided that the ship was in serious need of a major overhaul and was withdrawn from service in the same month for all of the work to be carried out.
Sir Arthur Rostron of the Titanic and Carpathia fame took command of her in late July 1920 and the ship was given her new name of Berengaria in April 1920 and the following year both the Begengaria and Aquitania were sent to Armstrong Whitworth shipyards to be converted from coal fired to oil firing.
The ship was named "Berengaria" after Queen Berengaria, the wife of Richard the Lionheart. The resonance being that Berengaria, Richard's wife, never set foot on the land she ruled as did the renamed ship never returned to the country where it was built. This was the first Cunard ship not to carry the name of a Roman province; the name still stayed with the tradition, however, of ships that ended with ia.
In May 1934 the Beringaria was in the headlines when like several of the Cunard liners, she ran aground on the mudbanks at Calshot on the Solent where she was latted pulled free from the mud by four Southampton tugs and the vessel suffered no damage and the incident did not affect her sailing schedule.
With the combining of the Cunard and White Star Line fleet in May 1934 her ex Hamburg-America sister ship Bismark - Now the RMS Majestic, Beringaria was no longer the fleet flagship as Majestic was larger, faster and newer.
The Berengaria served as flagship of the Cunard fleet until she was replaced by her sister ship, the , the former HAPAG liner Bismarck, in 1934. In later years, she was used for cheap prohibition-dodging cruises, which earned her the unfortunate nickname "Bargain-area". Towards the end of her service life she suffered several electrical fires caused by aging wiring and Cunard decided to retire her and sent her to the breakers in 1938. She was sold to Sir John Jarvie (who also purchased the RMS Olympic for the same reason) to provide work for his local region, where she sailed for the river Tyne under the command of Captain George Gibbons where she was scrapped down to the water line and left until after the war when the last remains were cut in two and then towed to T.W Wards yards where final demolition of the remaining hull pieces took place in 1946.