King will work alongside the agency's chief executive, Nick Simons, and be responsible for leading CST's planning team, as well as working on new business.
The planning director function was previously filled by Murray Chick, who remains the agency's chairman. King's appointment is designed to give Chick the opportunity to concentrate on the agency's brand consultancy division, Brand Aid, which he has been working on with Dave Trott.
King joins from Ogilvy Group UK, where he spent the past 18 months as a planning partner, working on the global Dove account and the agency's recently acquired Chiquita Bananas business.
Before Ogilvy, King worked at Lowe as the global planning director on Omo. His career also includes stints at M&C Saatchi, where he met Simons, and Grey. King previously spent nine years at Unilever.
Simons said: "My relationship with Stephen goes back a long way, and he was my first choice in what is a crucial role, so I'm delighted he's joining.
"With 17 years' experience under his belt on both client and agency sides, Stephen's knowledge will prove invaluable as we continue to attract the cream of talent to CST."
King added: "This is one of the most exciting opportunities in London. With unrivalled experience and a proven track record in advertising that actually works, CST has the perfect platform from which to move forward."