The moves follow a period of restructure at the end of 2007 that saw the agency moved more closely to its sister direct marketing agency Arc and the two creative departments brought together under the leadership of Jonathan Burley, the group executive creative director.
Mawhinney, who joined the agency from JWT in May 2005 and ran the Kellogg account, said he was leaving because he did not agree with the changes that were being made.
He said: "I've always found when an agency reforms and reinvents itself, it is not the most stable environment, and I couldn't give my best to Kellogg. I thought I'd give Jon the opportunity to build his own creative department."
Wood, who also joined from JWT and ran the Samsung business, has left because the Samsung account, which was previously split between London and New York, has now moved to the Chicago office.
Burley did not comment as Campaign went to press.
Mawhinney started his career in 1978 as a trainee art director at JWT. He left the agency after two years and worked at Bartle Bogle Hegarty, CDP, Still Price Lintas, Ogilvy & Mather and Faulds before joining JWT for a fourth time in 2002.
Wood joined Burnett in 2004 from JWT and before that, he worked at Lowe.