He takes the job title of executive vice-president and will be based in New York, starting on 1 May. Stewart will also sit on the firm's regional executive committee.
APCO CEO and president Margery Kraus said Stewart was hired to help the agency continue its post-buyout strategy of global expansion.
It completed its MBO from Grey Global Group in September 2004.
'Kirk's role is to make sure we're leveraging capabilities around the world,' Kraus said, adding that his client-side expertise would help APCO partner with clients at the highest level.
Kraus said that Stewart would help APCO serve multinational clients, specifically those from the Middle East and Asia that are trying to expand into western markets.
Stewart, who left Nike a year ago, had been much sought after in the interim. It is understood he was in the frame for the CEO role at Burson-Marsteller, his former employer.
APCO works with Nike on CSR issues, but both Stewart and Kraus denied that the appointment had anything to do with the companies' existing relationship.
Stewart joined Nike in 1997. Prior to that, he spent 16 years at Manning Selvage & Lee, rising to chief executive and president, and then chairman.