Howard Finkel for non-wrestling (hired 1975) :
http://www.imdb.com/name/nm0277885/bio
Jim Duggan for wrestler but not continuous (hired in 1987 but in WCW from 1994 - 2001).
For active wrestler, it might be HBK (hired 1988) since his time off from WWF/WWE seems less than all the yearly months off Taker (hired 1990) gets each year.