Entertainment Weekly reports that the Six Feet Under actress will appear in seven episodes of the ten-part run as PR guru Jilly Kitzinger.
The character will apparently recruit "the most important client of her career... and maybe of all time" and is described as "sweet-talking" with "a heart of stone".
Ambrose played Claire Fisher on Six Feet Under from 2001 to 2005 and also starred in short-lived Fox sitcom The Return of Jezebel James. Her film roles include 2009's Cold Souls and a voice part in Where The Wild Things Are.
She will join John Barrowman, Eve Myles, Mekhi Phifer and Bill Pullman in the new run, which began shooting on Monday.
Torchwood: Miracle Day will air on BBC One in the UK and Starz in the US.