[3] In the 1990s, the company expanded both domestically and internationally, forming WSP Asia in 1995, incorporating Graham Group and T P O'Sullivan and Partners in 1997, acquiring US practices Cantor Seinuk and Flack + Kurtz in 2000, as well as buying Jacobson & Widmark (J&W) in Sweden in 2001 (J&W was founded in 1938), LT Consultants Oyj and EMP Projects Oyj in Finland in 2003 and PHB Group in the UAE in 2005.

The offer was backed by WSP's board of directors as well as investors holding 37% of the company's shares, and the takeover was completed on 1 August 2012.

[9] In October 2014, WSP completed the purchase of New York-headquartered professional services firm Parsons Brinckerhoff from Balfour Beatty for US$1.24 billion.

[12] Together, WSP | Parsons Brinckerhoff is one of the largest professional services firms in the world with approximately 32,000 employees in 500 offices serving 39 countries.

[18] On December 3, 2020, WSP acquired Canadian-based geotechnical, earth-science, and environmental consulting firm Golder for $1.14 billion.

[21][22][23] In September 2022, RPS received a counter takeover offer from Tetra Tech for £636 million[24] with WSP opting not to increase its bid.