Mathers was previously chief operating officer at Blue Marlin Brand Design, and before that chief executive at The Brand Union.
His previous roles include managing director at Fitch, head of brand at Safeway, and president of the Design Business Association.
Mathers said: "I'm delighted to be joining a great brand like Holmes & Marchant and to be working with both the team there and the team at Media Square [the holding company for Holmes & Marchant]."
David Worthington, head of Media Square's design group, said: "I've known John for many years and am delighted that he's agreed to join us here at Holmes & Marchant.
"He's got an impressive mix of client and high-end agency experience and he brings great things to any company that he works for.
"Jon Davies has contributed so much during his 14 years working with Holmes & Marchant and we wish him all the best for the future."
Jon Davies said: "Holmes & Marchant is a fantastic business with a great heritage and a clear focused design offer. The business requires someone whom is as well versed in the industry as John to continue along its growth plan. I wish John and the team all the success for the future."
Holmes & Marchant's client list includes LVMH, Kraft, Unilever, Coty Fragrances and Heineken. It has been operating since 1967.