Pearson completed a financial restructuring at the end of 2012 and after discussions with chairman Jonathan Halford has now decided to move on.
Halford said: "We had discussions about the next generation. Mark has been here four years and now what's right for the leadership for the business going forward and the right thing for him and for the business."
The recruitment process has begun for a new CEO. Halford said Pearson may stay in the industry.
Pearson said: "I have truly enjoyed my time here at Gardman. It has been a challenging four years, as for so many businesses in our industry, but it has also been a great four years with my team at Gardman, supported by some excellent suppliers and working with some exceptional retail partners.
"As I said in December, I am delighted with the financial restructuring we completed last year; the business continues to generate good profits and cash and I know that as the Group moves into the next phase of its business development it will do so with a greatly reduced debt structure leaving it very secure for the future.
"As I now take this opportunity to move on with my own plans, I am confident that the Gardman Board will build a strong new leadership team under a new CEO to take the company forward, and I will, of course, stay close to support them over coming months as they make that transition."
Halford said: "I want to give Mark my huge thanks for the considerable contribution that he has made to our business over the last four years, and value greatly the support he will continue to give us as we bring in the new CEO and embark on the next stage of our plans."