Rainey, who last year stepped down from RKCR/Y&R after completing her earn-out deal with owner WPP Group, adds the directorship to others she already holds, including WH Smith.
Rainey said: "I am thrilled to be joining SMG and this impressive stable of brands at this exciting time of both challenge and opportunity in the media markets."
Rainey is also a founder and trustee of the volunteering charity Timebank and a trustee of the independent think tank, Demos. She is also a visiting professor at the University of Glasgow Business School and chair of the Marketing Group of Great Britain.
SMG chairman Chris Masters said: "I'm delighted to welcome MT Rainey to the SMG board. Her wide experience in advertising and brand management will be invaluable as we continue to develop and grow the SMG businesses."
Rainey stood down from RKCR along with co-founder Jim Kelly in October after 11 years.
Rainey, Kelly, Robert Campbell and Mark Roalfe completed their earn-out period in 2003 after the four sold the agency to WPP for £27m in 1999. She was still co-chairman of the agency, along with Kelly, until the beginning of this month when Roalfe took over as chair.
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