Although it generated more than $1 billion a year in sales, Ashland was a small player in the oil industry at a time when the cost of exploration was prohibitively expensive.

A new corporate strategy was implemented as Ashland now focused on refining and marketing and sought to grow its non-refining businesses.

In 1984, Ashland acquired Daniel, Mann, Johnson & Mendenhall (DMJM),[7] an architectural firm located in Los Angeles, California.

Originally focused on military projects, after World War II it had become one of the first integrated engineering and architectural firms in the western United States.

[9] The company went on to acquire a number of engineering, design and planning firms including engineering company Maunsell, urbanism and sustainability practice EDAW, Economic Research Associates (ERA), environmental management firm ENSR and The RETEC Group Inc., architects Ellerbe Becket[10] and Davis Langdon, the quantity surveyors and construction consultants.

On July 28, 2008, AECOM completed its purchase of Earth Tech Inc., a consulting and engineering firm, from Tyco International for $510 million.

[15][16] On July 14, 2010, AECOM announced its acquisition of Tishman Construction Corp., a leading provider of construction management services in the United States and the United Arab Emirates, in a $245 million transaction including $202 million in cash and the remainder in AECOM common stock.

[17] On July 13, 2014, AECOM announced its acquisition of URS Corporation, an engineering, construction, and technical services firm for US$56.31 per share in cash and stock.

[22] In October 2019, AECOM announced plans to sell their Management Services division to private equity firm American Securities LLC and Lindsay Goldberg for $2.405 billion.

[25] In October 2020, AECOM announced the sale of its Power construction business to private equity firm CriticalPoint Capital.

[26] In August 2021, AECOM announced plans to relocate its global headquarters from Los Angeles, California to Dallas, Texas.

[29] AECOM's first president and CEO was Richard G. Newman, who came to Ashland through its acquisition of Daniel, Mann, Johnson & Mendenhall (DMJM).

At the same time, AECOM said that the board of directors would be expanded to include representatives of activist-shareholder Starboard Value, the fifth largest shareholder.