Malone joins Feed as its first director of social content and will sit on the management team, alongside: Matt Lynch, the managing director; Rob Armstrong, the chief executive and Simone Micheli, the executive creative director.
He will not take a stake in the business and will report to Lynch and Armstrong.
Malone sold Grape, the social media agency that he co-founded in 2009, to The Karma Communications Group, the owner of Karmarama, in 2013 and stayed on at the agency as managing director. In 2014 Karma re-branded Grape as Krowd.
Lynch said: "This is a pivotal appointment for Feed. We've landed one of the most established and respected content and social specialists in adland.
"Among the self proclaimed gurus and experts he stands out with an impressive track record. From the moment he walked through the door it was clear Mark¹s ambition fits our own.
"We plan on building the most talked about social and content division in London. We invite brands who are sick of the same old social solutions to come and see us."
Earlier this year, eBay picked Feed as its European CRM agency. Feed, which is independently owned, also works with Gumtree and The Telegraph.