Livesey, 31, who is currently the head of account handling at HHCL/Red Cell, will fill the role on M&S previously occupied by James Murphy, before his promotion to the position of managing director in March 2003. She takes up the post next month.
Murphy said: "Tanya is a great addition to our new management line-up. She has been instrumental in the strongest creative work and she leads by example."
Livesey has been at HHCL for four years, where she has managed the Molson Export, AA and Pot Noodle accounts, among others.
In 2001, she was promoted to become a partner. More recently, she ran the Sky account and was responsible for output including the "odd couple" campaign for Sky Plus, starring David Hasselhoff and Mike Reid.
Livesey was named the joint best account handler at the IPA's 2003 Best of the Best awards, for her work on Pot Noodle's "slag of all snacks" campaign.
Before her four-year spell at HHCL, Livesey was at Lowe Howard-Spink, which she joined as a graduate trainee. She rose to become a junior account director on accounts including Weetabix, Stella Artois and Heineken.
Livesey said: "RKCR/Y&R has a strong creative reputation and an extraordinary client list. Joining their ambitious new management team was an opportunity too good to miss."