The former ER star, who recently had a role in Doctor Who, will play Blanche Mottershead - the younger sister of Dame Eileen Atkins's character Lady Holland.
"I simply couldn't resist the opportunity to get inside the iconic 165 Eaton Place," Kingston told the BBC.
The news follows Atkins's announcement that she would not be returning to Upstairs Downstairs for the new series, for which filming begins in October.
The new series of the BBC One costume drama is set in 1938 and deals with the months leading up to the outbreak of World War II. Actor Kenneth Cranham will also join the cast.
"Outside 165 Eaton Place, London and Europe are poised on the knife-edge of calamity," writer Heidi Thomas said. "Inside, there's a whole world of dark and dangerous emotions to explore."
Kingston added: "Heidi really has created a wonderfully intriguing character in Blanche who will over time reveal some secrets of her own."