The agency secured the business following a three-way shoot-out against Proximity and Joshua G2.
M&S has previously created some of its direct marketing in-house, and has also used agencies on a project-by-project basis.
CMW will now take on responsibility for all of M&S's below-the-line consumer communications across its range of services including its &More credit card and online shopping facility.
It is understood that the campaigns will range from loyalty and acquisition programmes to promotions of new store openings. The appointment follows a restructure of M&S's marketing department in March, which saw the departure of the marketing and communications director, Jude Bridge.