In the 1940s, self-unloaders would bring new business to ASC, and the focus of the company would shift from transporting iron ore and grain to shipping coal and limestone.

The company again launched an expansion effort and used funds provided under the Merchant Marine Act of 1970 to retire older tonnage and begin construction of a new fully modernized fleet of ten vessels.

In 1973, the Boland and Cornelius families sold ASC to the General American Transportation Corporation (GATX), which oversaw completion of the newbuild program.

On February 7, 2020, GATX announced on their corporate website that an agreement for the sale of American Steamship Company had been reached and was pending regulatory approval before proceeding.

On May 17, 2020 it was announced that the buyer was American Industrial Partners (AIP), a hedge fund group that had purchased RAND Logistics Inc and their subsidiary fleets of Grand River Navigation and Lower Lakes Towing as they emerged from bankruptcy proceedings.