Kempton joined the Chamber Orchestra in 1995 as general manager, after nine years with the Scottish Chamber Orchestra. During his tenure in St. Paul, he has been the chief planner of the orchestra's tours.
"During his time at the SPCO, by far the longest of any person in the position, Barry has brought grace, charm, knowledge, friendliness and commitment to his work," orchestra president and managing director Bruce Coppock told the paper.
The orchestra will begin a national search for Kempton's replacement shortly.
The City of London Sinfonia was founded in 1971 by conductor Richard Hickox.