Doug Benson will replace Rogers & Wells current executive director Richard Killian when he steps down from the post on millennium eve - the day before the new merger comes into effect.
Killian will stay on as a consultant to Rogers & Wells, where he has been based for nine years.
The role of executive director will change when Benson takes over as it will no longer be autonomous and Benson will report to Garth Pollard, the new chief operating officer of the merged firm, who will be based in London.
However Benson, who is by profession a management consultant, will be responsible for putting the merger into effect in the US.
Benson says: "I had been at Shearman & Sterling for seven years and I was very excited about what is happening in the globalisation of law firms and the merger between Clifford Chance & Rogers and Wells presented a great opportunity.
"I received a call from an executive recruiter and thought that there is nothing else like this in the world and to be in on it at the beginning is very exciting."