Cake also announced that Jim Dowling will take the role of managing director. The new appointments will be effective from 1 December.
Mathieson is to return to the music industry with new consultancy Harmonic launching in January. He will work alongside Macky Drese, former vice president of brand partnerships at MTV and Viacom International Networks.
Mathieson has been developing his new venture for the past few months and recently activated Virgin Atlantic’s on-board #Flightdecks stunt, featuring performances from Gorgon City and Rudimental. He was previously head of PR for Virgin Records before founding Cake with Pettett and Mark Whelan in 1999.
Pettett said: "While I am extremely keen to capitalise on the energy we have at Cake, there is no doubt, we will all miss Mike greatly, both for his drive and vision over the past 15 years as well as his infectious enthusiasm for the business. I know all the teams within Havas join us in supporting him in his new venture with his first love – music.
"For us at Cake it is a real opportunity to build on the momentum we have generated over the past year with a string of high profile new clients and awards. These are exciting times at Havas as it continues to invest in data and content, and the opportunities for Cake to play a front and centre role in a truly joined up network are greater than ever."
Mathieson added: "We co-founded Cake on four basic principles: to be creative, have fun, make some money and that’s it. Fifteen years on, I think I have finally achieved all four of those principles and I’m excited to be making a return to the music industry. Of course I am sad to leave the business that I founded, but I leave it in the safe hands of my long-time partners and the Havas Group."
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