United States Lines came into being after the United States Mail Steamship Company had failed to successfully operate Government owned ships on the North Atlantic after World War I. The U.S. Shipping Board took over U. S. Mail Steamship Co operation of surplus ships after it had incurred heavy losses since beginning operation in 1920.

Without a break in service the all the ships were then run under United States Lines which was formed in August 1921. The former German liners Amerika and George Washington began sailing New York - Bremen, with the Centennial State on the New York - London service. In 1922 two "535" class refitted transports were added to the fleet. Peninsular State and Lone State were renamed President Pierce and President Taft, but within weeks they were renamed President Roosevelt and President Harding. The "502s" from U.S. Mail S.S. Co. were renamed President Van Buren, President Adams, and joined by still more "502s" which became President Polk, President Garefield. The Princess Matoika (formerly known as Princess Alice) and Hudson became President Arthur and President Fillmore under United States Lines.

In 1923, four of the ships were sold and the huge Leviathan, (formerly known as Vaterland of Hamburg Amerika Line) began sailing the New York - Southampton service after a very extensive rebuilding. She became the flagship of the United States Lines fleet. Late in 1923 the five remaining "502"s were sold by the US Shipping Board to Dollar Line. To offset the loss, five vessels built at Hog Island were refitted to carry passengers in the New York - London service; American Merchant, American Shipper, American Banker, American Farmer and American Trader all began service in 1924 under American Merchant Line. The Republic, another former German Liner also entered Atlantic service.

In 1926 Amerika suffered a serious fire at Newport News and was out of action for a year. Debts continued to mount under United States Lines management and bids were called from private owners.

P.W. Chapman Co successfully bid in March 21, 1929 $16 million for eleven ships. Stipulated in the agreement was the fact that Chapman must continue the service for not less than 10 years, and build two 45,000 ton ships as consorts for Leviathan. The USSB would lend 75% of the cost. Chapman traded under the name United States Lines Inc., of Delaware. Two three stacked liners were designed by T. Ferris, but the introduction of Norddeutscher Lloyd Line's Bremen made it obvious that speed was more important. Two newly designed ships with four screws and two funnels were planned, but after the stock market crash in October were abandoned in favour of two smaller ships.

Falling revenue caused the Amerika and George Washington to be laid up, and the Republic was handed over to the War Department for transport work. In October 1931 Chapman foreclosed on the Shipping Board. Leviathan, President Roosevelt, President Harding, the two scaled down ships under construction, and two newly acquired transports were all sold to the newly formed United States Lines Company, of Nevada. A subsidiary of International Mercantile Marine Co which also bought the American Merchant Line and added them to the United States Lines list of companies.

In August 1932 the Manhattan, United States Lines' first purpose built passenger ship entered service, her sister Washington followed in May 1933. They entered service in economically depressed times and their appearance precipitated the withdrawal of Leviathan. The Shipping Board demanded she be put back in service, but after more losses were incurred, she was laid up again in 1934.

A new generation of ship appeared with the 34,000 GRT, Gibbs & Cox designed S/S America of 1940. The War in Europe kept the Atlantic liner in U.S. coastal waters. Six of the seven former American Merchant fleet ships were lost to enemy action after they had been transferred to the Belgian flag, under IMM control. Four replacements were laid down in Pascagoula but they all became Army transports. Remaining ships were requisitioned and put into Navy service; President Roosevelt, Manhattan, Washington and America became U.S.S. Joseph T. Dickman, Wakefield, Mount Vernon and West Point. In 1943 United States Lines of Nevada was merged into IMM and the name became United States Lines Company, of New Jersey.

Wakefield was laid up in 1946. Joseph Dickman was laid up and scrapped. Washington was returned to one class service briefly to bring war brides and children to the United States. She briefly became a consort for S/S America before being recalled to Military service until 1951. America returned to the Atlantic service with her first sailing in November 1946.

With great Post war demand for shipping United States Lines began operating C-4 type transports as a means of mass transportation for displaced people. The Line also began building up a large fleet of cargo ships, all with the name American as a prefix, freighters in Australia service were given the prefix Pioneer.

In July 1952, the S.S. United States made her maiden voyage New York - Le Havre - Southampton. At 54,000 GRT she was the largest and fastest liner ever built in the U.S. On her maiden voyage, she broke Blue Riband speed record that had been held by Britain's R.M.S. Queen Mary for the fourteen years. The United States became the first American title holder for the Blue Riband in a century. In 1955, a sister for the United States was proposed but nothing came of the plan. America and United States maintained a transatlantic service for twelve years until competition from air traffic and rising costs forced the sale of America to Chandris Line in 1964. The United States, without a running mate incurred ever bigger debts thus was withdrawn from service in 1969 and laid up. This marked the end of American flagged service on the North Atlantic.

After the termination of passenger services, United States Lines survived as a container shipping line. In 1982 they purchased Moore-McCormack Lines. They then bought Delta Lines from Crowley Maritime (who had acquired the company in 1982) in 1985. However United States Lines filed for bankruptcy in 1986, due to financial difficulties caused by too rapid expansion. The service was gradually phased out between 1986 and 1989 and the company was formally liquidated in 1992.