Mendelsohn, the executive chairman of Karmarama, joins Ben Bilboul, the group chief executive of Karmarama, on the board.
Filings at Companies House show Mendelsohn became a director of the company on 18 February.
Karma was created following investment in Karmarama by Phoenix in June 2011. Other agencies in the group include Kream, Kaper and Acquity Customer Insight.
Other board members include the group chairman Charles Watson and Justin Hicklin, the founder of Crayon, which was acquired by Karma in January 2012 and subsequently merged with Karmarama in May 2012.
Mendelsohn, the current IPA President, joined Karmarama in 2008 as a partner. She was a previous deputy chairman at Grey and also worked at Bartle Bogle Hegarty.
This article was first published on campaignlive.co.uk