Assigned to the route between Liverpool and New York, Celtic experimented with a mode of slower than usual rotations, but was also used for a long cruise in 1902 which met with some success.

However, this commercial career ceased on 10 December 1928, when, in stormy seas as she approached Cobh, Celtic grounded on the rocks.

All the passengers were rescued, but the company considered it futile to attempt to salvage the liner so Celtic was scrapped on the spot.

Celtic's conception, as the other Big Four-class ships, came about in 1898 following a noticing of an opportunity to introduce a series of liners which would prioritise more on comfort rather than speed.

This was accomplished in comparing the net tonnage (a measure of a ship's total enclosed space) between their two newest North Atlantic Liners; SS Cymric, which had entered service in February 1898, and RMS Oceanic, which was nearing completion at Belfast.

[6] The liner met all new objectives: with it, the company definitively abandoned the race for speed and never again sought to win the Blue Riband, favouring comfort and economy.

[7] Celtic was designed to operate an average speed of 16 knots, which was what White Star ships already reached almost thirty years earlier.

The ship was also provided with four masts which were not intended to receive sails (but served as support for the cables of the wireless telegraphy and the lookout's nest)[10] Although slightly smaller, Celtic offered a luxury equivalent to her sister ships (although Adriatic benefited from new facilities such as swimming pool).

On one occasion, in September 1904, she carried on board 2,971 passengers, the largest number of people ever embarked in peacetime on a ship of the White Star Line.

Her size was such that her loading time in each port was one week, in order to allow the embarkation of coal, goods, and passengers.

[8] In February 1902, the liner was sent to the Mediterranean Sea for a five-week cruise, with 800 passengers on board, which allowed the White Star Line to experiment with this means to make its ships profitable during the off-season.

[15] On 25 December 1905, she encountered a rogue wave that hit the ship and caused damage to the second class facilities, in particular, glass panels in the smoking room, taking with it a door, and frightening the passengers.

[6] In addition to the other Big Four-class liners, her success inspired the construction, again by Harland & Wolff, of Amerika of the German company HAPAG, which copied her profile and her characteristics, while being slightly faster and above all more luxurious.

[9] During the following summer, it was the White Star's turn to open its own service from Southampton, with its express ships, Majestic, Teutonic and Oceanic, which were quickly joined by the last Big Fourliner, Adriatic.

[17] At the start of World War I, Celtic was one of four White Star Line ships that were converted to auxiliary cruisers, along with Cedric, Oceanic and Teutonic.

However, the British Admiralty quickly became aware of the uselessness of such ships, too large to ideally fulfill the missions entrusted to them.

The same year Celtic lost a propeller in Boston Harbor and had to return to the dock, forcing her passengers to leave by train for New York in order to take another ship.

[23] The second collision took place on 29 January 1927, when Celtic was rammed in thick fog by the American Diamond Lines Anaconda off Fire Island.

On December 1, the liner left New York for Cobh, via Boston, with 300 passengers on board, 350 members of the crew, and a large cargo.

Some time later, the weather seemed to improve, and the captain changed his mind and returned to Cobh to try to disembark the passengers and bags of mail that were to stop there.

A salvage team from Cox and Danks was provided to attempt recovery, but several men died after a hold loaded with grain and flooded with seawater was found to have filled with toxic fumes; due to structural failures it was judged the ship could not be moved or salvaged, and was abandoned to the insurance company who declared the ship to be a total loss.