During the first half of the 20th century, William Halcrow led the business into new avenues of civil engineering, including deep tunnelling and hydroelectric dams.

[4] The company was called on for various projects during the Second World War, this included the design and supervision of the construction of deep tunnel shelters, military ports, and the Mulberry Harbours (used in the D-Day landings).

[14][6] In the aftermath of the Aberfan disaster, Halcrow was engaged by the National Coal Board to monitor the condition of various colliery spoil tips across the country.

[7][15] The company's overseas work at this time included a wide range of roads, bridges and harbours in countries such as Ghana, Libya and Mozambique.

[19][4] By 2000, Halcrow Group's British-based projects accounted for roughly 60 percent of the company's turnover, while the remainder was derived from its undertakings outside of the British Isles.

Its customers included government departments, public sector authorities and utilities, industrial and commercial companies, international funding agencies and financial institutions.

[21] It was subsequently reported that Halcrow was effectively rescued by CH2M Hill, having incurred a pre-tax loss of £71m in its last year of independent trading (to 31 December 2011), on a turnover of £238m, down from £331m in 2010.