He is the second managing director to depart the agency this year. Tim Greatrex left after missing out on the chief executive job to Antony Young, who was brought over from Asia to run Zenith's UK operation in February.
Turzynski resigned from ZenithOptimedia after talks with Young.
Gerry Boyle, one of the agency's deputy managing directors, will replace Turzyn-ski. Boyle, 32, joined the agency as the head of planning from Michaelides & Bednash.
Turzynski's future plans are unclear but he has not gone straight into another job at a media agency.
Mark Waugh, another deputy managing director, will also take responsibility for ZenithOptimedia's Wigmore Street operations.
The agency will retain its three account groups; headed by Boyle, Waugh and the fellow deputy managing director Tom George.
All three report directly to Young, who, with the commercial director, Greg Grimmer, make up the agency's UK executive management team.
Young said: "Greg has been a valuable member of the management team."