They will work together on projects under the banner GMW/Interior Architects. The arrangement, known as an affiliation agreement, allows the firms to pool resources while remaining separate entities.
GMW senior partner Lyn Edwards said that the deal, finalised earlier this month, would provide IA with a European network. GMW would work as a consultant on IA's European projects and enter other overseas markets.
Chicago firm IA has sent three members of staff over to GMW's offices in Kensington, west London.
Edwards added: "It gives us access to contacts in Asia and the Pacific, and Interior Architects gets the benefit of our contacts across the European Union in the European Architects' Alliance."
GMW is a founder member of the European Architects' Alliance, a network of affiliation agreements with six European architects.
The alliance, which includes architects from France, Denmark, Poland, Italy and Ireland, aims to forge links in Germany, Spain, Portugal and Greece.
It gives us access to contacts in Asia and the Pacific
Lyn Edwards, GMW partner
GMW is in talks with Greek practice KLMF Architects & Associates over an affiliation agreement that would enable it to use its Athens office. The firm is also interested in eastern European countries that are joining the EU.
The firm plans a similar consortium to the European Architects' Alliance with practices in the Middle East and Africa. Architects in the United Arab Emirates have been targeted.
GMW has also forged links with practices in southern Africa, which it hopes will be part of a future framework. GMW is one of four on the Foreign and Commonwealth Office's framework of practices providing fit-outs in the continent.
The practice is currently working with South African firm EHH on the construction of a deputy high commission in Cape Town.
It is also liaising with DAJ Architects in Mozambique over a £100m spa hotel in Ponto D'Ouro, and with a South African architect on a 93,000 m2 shopping centre in Lenasia, near Johannesburg.