King, 39, spent his entire 13-year playing career with Tottenham and has been a club ambassador since retiring in 2012.
"Ledley will join as a member of our first-team staff, working alongside the coaches, analysts and playing squad both at the training centre and on matchdays," Tottenham said on their website on Thursday.
"He will also take on the role of supporting our academy players as they look to step up to the first team."
Former centre-half King made 323 appearances for Spurs after making his debut in 1999 and lifted the League Cup as skipper in 2008.
Mourinho said: "I'm extremely pleased to be welcoming Ledley into the first-team group as we continue our preparations for the new season.
"He has a tremendous affinity with this football club and the fans appreciate just how much he has accomplished as a true Spurs man both on and off the pitch."