Mr. Topolewski is already the chief creative officer at the General Motors Corp. agency, which handles the automaker's Cadillac and Pontiac brands.
Roger Haupt, president and chief operating officer of parent Publicis, previously was the agency's chairman since its formation in March. Chemistri said the change frees Mr. Haupt to handle his duties at the holding company level.
Chemistri will continue to report to Mr. Haupt. Patrick Sherwood remains Chemistri's president-CEO.