He will also be responsible for overseeing the group's expansion as it extends its service offering into corporate branding and engagement programmes.
Mathers joins from The Brand Union where he has been chief executive since 2002. Prior to this, he held various roles on both agency and client side, including managing director at Fitch, client director at Brand New, which is part of the Michael Peters Group, and head of brand at Safeway.
Andrew Eyles, chief executive of Blue Marlin, said: "[John's] extensive international experience and track record in building and leading a global network will be invaluable as we take Blue Marlin to the next stage of its development.
"He brings an in-depth understanding of the challenges facing our industry, and a proven track record of commercial astuteness and effectiveness with both clients and colleagues, as well as the wider brand design and creative community."
The appointment is a part of Blue Marlin's strategy to move to operating as one global unit as it seeks to expand its international network of offices.
Mathers said: "It's very exciting to come on board at this stage in Blue Marlin's evolution. With its international perspective, unique global structure and substantial investment in creative output, Blue Marlin is well placed to deliver for clients who are increasingly looking for the big idea. I am looking forward to working with the team."