San Francisco-based Thelen Reid & Priest and New York's Brown Raysman Millstein Felder & Steiner have confirmed they are to merge, creating a 250-partner, 617-lawyer national firm with an estimated $400m (£210.21m) annual turnover.
The firm will be known as Thelen Reid Brown Raysman & Steiner and will be co-chaired by Thelen Reid chairman Stephen O'Neal and Brown Raysman managing partner Julian Millstein.
The firm will conduct a comprehensive compensation review in the New Year to determine equity status for the new partners. The combined turnover would have ranked the merged firm at 66 in The Lawyer Global 100 2006, just ahead of Bryan Cave.
The merger will give the firm 255 lawyers in New York and 242 in California, primarily in San Francisco and Los Angeles. The merged firm will also have a 70-lawyer Washington DC office, as well as the Shanghai base that Thelen Reid opened in late August.
The merger gives each firm the national coverage it had been after. Thelen Reid, despite the 1998 merger between Thelen Marrin Johnson & Bridges and Reid & Priest, had only 78 lawyers in New York.
In 2003 Thelen Reid formed a strategic partnership with Masons, before it merged with Pinsents, to create Masons Thelen Reid, a specialist construction, engineering and infrastructure practice.
Masons' construction team has worked with Thelen Reid on a number of projects around the world. Masons Thelen Reid is allied with Baghdad-based Salam Abdullah & Partners. Salam Abdullah is an Iraqi lawyer who returned to Baghdad in January 2004 to set up his law firm.
Tony Bunch, head of international development at Pinsent Masons, said: "I think it's a good move as we perceive it, insofar as they'll bulk up in New York.
"From our point of view, the relationship with Thelan Reid is based heavily on the infrastructure and construction side. Adding a media and IP/IT capacity presents the opportunity to widen the relationship."