He will part company with the Bombardier ale brewer on May 31.
Lewis said: “It's never easy to leave a company where you have so much time and respect for the people and the brands, but there comes a point where you know the time is right to seek out new challenges.
“I'm very proud of my achievements at Wells & Young's and leave the business and the marketing output in great shape.
“I'd like to thank everyone at the company for all their support over the past five years and to wish them every success in the future".
Managing Director Nigel McNally added: "I'd like to thank him for his contribution to our business and, particularly, developing the recent Bombardier campaign. We wish him well for the future."