Key led his county to one Twenty20 Cup crown in 2007, as well as one other T20 final and the Friends Provident Trophy final.
He took over as captain from David Fulton at the end of the 2005 season.
Earlier that year, Key had made what has proved to be the last of his 15 Test appearances for England.
Key, who also played in five one-day internationals between 2003 and 2004, made the last of his England appearances in a one-off Twenty20 clash in 2009.
The 33-year-old has represented Kent since playing at age-group level aged 11. He made his first-class debut in 1998 aged 18 and has since played in 253 matches for Kent.