She will take up the newly created role of US managing partner in April and run Brunswick's three US offices - New York, Washington and San Francisco - alongside senior partner Steve Lipin.
Her London role will be assumed by Brunswick partner Nick Claydon, the ex-comms head of British Airways.
Gilchrist confirmed the move and commented that it was ‘testament to how important the US market has become to Brunswick'.
The agency once again topped the global M&A by value league table in 2009, with a total of 106 deals worth US$321.7bn (£199.7bn) and recently worked with Kraft Foods on its high-profile takeover of Cadbury.
Gilchrist has been senior partner in London for four years and was previously retail correspondent at The Times and also worked as a management consultant at Bain & Company.