Partners at San Francisco's Thelen Marrin Johnson & Bridges and New York's Reid & Priest have voted in favour of merging the two firms, paving the way for an official launch on 30 June.
The new firm, called Thelen Reid & Priest, will have almost 350 lawyers 190 from Thelen Marrin and 160 from Reid & Priest spread over San Francisco, New York, Washington DC and Los Angeles.
The firms are describing the tie up as the first bi-coastal US merger involving major firms.
Jonathan Siegfried, a Reid & Priest partner who will sit on the firm's executive committee, said: “All of our business dealings have become more national and international and, to serve client needs, it is important for us to offer a seamless national service and position ourselves to provide services on a global basis.”
For Thelen, which is best known as a litigation firm, the tie-up will boost its presence in corporate finance, a practice area where Reid & Priest has made its mark, notably in utility and energy industry financings.
The firm will also strengthen its position in the key New York market, where a number of rival Californian firms have built up large practices.
The merger will boost Reid & Priest's profile on the West Coast, a market which other New York firms have found difficult to crack.
A further benefit for both firms is that they will be able to invest more in international development. Neither currently has offices outside the US.
Siegfried said it was premature to be talking about specific plans, but added: “Clearly one of the issues we will explore is how to service even better our international needs.”