Marshall left the Starcom MediaVest Group in September after 28 years at the company.
Danielle Klein, former head of international sales for CBS Outdoor, is also joining Media By Permission as consultant sales director.
Media by Permission managing director Mike Baker said: "There are very few people in media who have the respect that Jim Marshall still enjoys after more than two decades in the media business.
"He has a black book second to none and a great passion for the industry, which makes him an ideal colleague."
Marshall said: "The concept of Media by Permission is attractive because it brings efficiency and relevance to both sides of the business, adding value to buyers and sellers in equal degrees. It is exciting to be working on a small business startup.
Media By Permission was set up as a CRM system as a customer relationship management system to connect media owners and media buyers in July last year."