Jonason, best known for his work on the highly acclaimed Diesel account, has joined as creative director and head of the new agency, called Made.
He is an equity partner in the venture.
Made has opened its doors with three founding clients - the Italian fashion brand Stefanel, Bjorn Borg underwear and the designer J Lindeberg. Stefanel was a Euro RSCG client, having appointed the agency to its pan-European business last year. Jonason said that he intends to handle diverse clients from mobile phones to jeans.
Simon Moore, a former consultant from Agency Assessments International, has joined Jonason as a founding partner. Emma Trotman, the erstwhile marketing director at Euro RSCG, and the art director Tove Langset join as the agency's first employees.
Jonason said he had been in informal discussions with Burnett's worldwide chief executive, Steven Gatfield, about establishing a joint venture agency since last summer. "I was waiting to see if my family were settling in England before I decided to take the job,
Jonason joined Burnett to work on the new agency in January, having left his creative directorship at Euro RSCG when it lost the £40 million Citroen account to Partners BDDH.
Jonason said Made would initially specialise in working with medium-sized clients but because of its links with Burnett's global network would be capable of operating on the international stage.
Jonason first made his name working on the Diesel account in his native Sweden. The much-acclaimed ads were created by the Swedish agency Paradiset DDB, of which he was a founder member and creative director. He had previously been the marketing director of the clothing company Hennes & Mauritz.
Before joining Euro RSCG, Jonason was one of the founders of the ill-fated global services specialist agency Cave Anholt Jonason. Last June the agency closed its doors after only eight months following personality differences between the partners.
Gatfield is attempting to improve Burnett's group offering in London and appointed Bruce Haines as group chief executive as part of this drive earlier this year. Haines said: "Made is a fantastic addition to the group and complements my plans for Leo Burnett UK and its extended offering."