Kingston will play the younger sister of Lady Maud Holland, played by Dame Eileen Atkins in the first series which aired over the festive season.
Last month Atkins announced she would not be returning to the series, with reports that she was unhappy with the direction the scripts were taking.
Returning for the new run are Ed Stoppard as diplomat Sir Hallam Holland, Keeley Hawes as his wife and new mother, Lady Agnes.
Jean Marsh returns as Housekeeper, Mrs Rose Buck; Anne Reid, Claire Foy, Art Malik, Blake Ritson, Nico Mirallegro, Adrian Scarborough and Neil Jackson.
The six new episodes, due to air next year, are set two years on, in 1938 when Chamberlain is in power.
Kingston says: “I simply couldn’t resist the opportunity to get inside the iconic 165 Eaton Place and join a show steeped in such history but that has been revived for a whole new generation.
“I will be playing the uncompromising, straight speaking spinster, Dr Blanche Mottershead, the younger sister of the late Maud, Lady Holland and Sir Hallam’s Aunt.”
Kenneth Cranham also joins the cast as Sergeant Ashworth, a police officer who answers an emergency call at 165 Eaton Place.
Having fought on the front line in the Great War, he finds Mr Pritchard’s past as a conscientious objector appalling and torments him about it, setting off a chain of events which leads to near collapse downstairs.
Other new characters set to cause drama, intrigue and passion among the household in the new series are Caspar Landry, a charismatic, American, multi-millionaire, introduced to Lady Agnes by Joseph Kennedy who is instantly charmed by her.
Lady Agnes is taken with this refreshing, vibrant American, who offers her a wealth of hope and possibility in a dark and fearful pre-war London…
Writer Heidi Thomas, says: “The really exciting thing for me about Upstairs Downstairs is the way in which public events and private lives intertwine. Outside 165 Eaton Square, London and Europe are poised on the knife-edge of calamity. Inside, there’s a whole world of dark and dangerous emotions to explore.”
Buy Upstairs Downstairs series one on DVD from Amazon.co.uk, HMV.com and Play.com