Spong previously held a media consultancy role at Royal Mail, which has just wrapped up a review of its £25 million account by rehiring its nine-year incumbent, Carat.
He has also worked on the account management side at direct marketing agencies including Proximity.
Kerry Glazer, the chief executive of the AAR, said: "Tony's special blend of skills and experience means he brings a deeper understanding of client and agency requirements during the search and selection process - a factor critical to the AAR's business approach and delivery."
Constable revealed that she was to leave the AAR to join a rival intermediary service, Agency Insight, last month. She joins as a senior consultant, with responsibility for developing its direct marketing and digital sectors.
The AAR works with clients that include the BBC and Christian Aid. It also works in areas such as advertising, design and customer magazines.