The Doctor Who actress will play Dr Blanche Mottershead, the younger sister of Dame Eileen Atkins's alter ego Lady Holland.
Kingston told The Sun: "I can say the character is very complex, very intriguing and very straight-talking. She has some very dark secrets of her own."
Blanche was brought in after Atkins announced her departure from the period drama after one series.
Upstairs Downstairs will return to BBC One in spring 2012.
Watch the original trailer for the first three episodes of Upstairs Downstairs below: