The move follows a review in which Kirshenbaum beat out Omnicom's GSD&M, Austin, and independent Creature, Seattle.
Billings were not disclosed. Agency representatives did not comment.
David Schembri, vice president of Mercedes Benz USA, declined to comment but did say that the company has hired agencies for various projects and brand positioning is one of three key areas Smart is focused on; the other two are product definition and establishing a retail network of dealers.
Kirshenbaum has never handled brand positioning for an automaker, though in the early 1990s the agency created ads for Tri-state Acura dealerships. The agency's creative reputation is for off-beat advertising, appearing in traditional and nontraditional media. Mercedes is most likely looking to apply the same nontraditional buzz-building tactics employed by rivals BMW for its Mini Cooper and Toyota Motor Sales USA for Scion.