Hatton, who has worked at BBH for five years on brands including ITV, Lynx, One2One and Barclays, will report to Guy Murphy, whom she succeeds.
Murphy was promoted to deputy chairman last year, but continued his planning role until a suitable replacement was found.
Hatton was appointed to the board one year after joining the agency.
In 2002, she led BBH's Grand Prix-winning entry to the IPA Effectiveness Awards for Barnardo's. Last year, she co-wrote the Lynx Pulse paper that was awarded at the Marketing Society and IPA Effectiveness Awards.
Murphy said: "Rachel has been an important part of BBH's planning success during the past four years and she will do a great job managing the department."
Heather Alderson, also a board planner at BBH London, has been promoted to the new role of director of international planning.
She will report to Murphy on UK accounts and to Emma Cookson, the global head of planning based in New York, on other business.
The creation of the role follows BBH's expanding international client list. Last year, it picked up a global brief from Vodafone, and extend its global Unilever relationship.