Quorn is the market leader in the £582 million meat-free market, with a share more than twice the size of any of its competitors.
Walker will work alongside St Luke's, which won the creative account two years ago and repositioned Quorn away from the vegetarian food sector and towards mainstream healthy eating.
However, the Advertising Standards Authority banned the company from referring to the meat substitute as a "mushroom protein".
Pete Banks, the commercial director of Marlow Foods, said: "Our brand has huge potential for further growth, and we are well placed to continue our current brand growth of 5 per cent year on year."