Chicago police said the incident remained a "death under investigation".
Tilley, who had been at the agency for 11 years, took over as the managing director for creative in September 2006. At the time, he called it a "dream job" and set about trying to restore flagging growth at the agency, whose clients include McDonald's and Anheuser-Busch.
Rick Carpenter, the DDB Chicago chief executive, described Tilley as "a mentor to many and a friend to all". He added: "The reality of this news is difficult to comprehend."