Formerly director of comms for North America, Stewart will be based in
London where he will report to McKinsey managing partner Ian Davis.
He will also take temporary control of comms for the UK, Middle East, and part of Europe, covering the role left vacant last week by Tony Danker.
Danker has taken a 12-month career break and is to study a masters degree in public administration at the Kennedy School of Government at Harvard.
External relations manager John Cheetham said it is not known whether Danker will return to McKinsey in a comms capacity.
McKinsey is on the hunt for Danker’s replacement. In New York, external communications manager Anne Board has replaced Stewart.