The Philadelphia, Pennsylvania-based company was the largest American shipping company during its existence, rivaled only by Baltimore-based Atlantic Transport Lines, although all American oceanic shipping concerns were dwarfed by British companies such as the White Star Line and Cunard Line and German ones such as HAPAG.

Its first president was Herman Joseph Lombaert (1816–1885), a PRR vice-president who resigned from the railroad when the shipping company became independent.

In the IMM, the American Line generally handled traffic between the United States ports of Philadelphia and New York City and the British ports of Liverpool and Southampton.

Sister company Red Star Line handled traffic between America and the European continent, primarily through Antwerp, Belgium.

When the IMM trust failed in 1932, the American pieces of the combine were once again solely under the American flag, this time in the guise of United States Lines.