The decision to review the account was made by design and marketing director Anjum Ahmed, who took over the marketing role from Nick Booth in January.
Magnet came up with a four-strong shortlist of MediaVest, Universal McCann, PHD and MediaCom.
Ahmed said: "In the last few year,s we have taken a strategic change of direction and moved both the advertising and PR accounts, which has seen some excellent success. We felt the time was right to also investigate alternative media strategies."
The retailer's advertising account is handled by CheethamBell JWT and its PR account is handled by Brazen.
Magnet runs a variety of above and below-the-line activity to support its four major sales periods each year and increase sales at its 164 stores nationwide.
MediaVest Manchester last month won the £12m Barratt Development account in conjunction with MediaVest London and the £1m account for Hain Celestial, the owner of the Linda McCartney vegetarian food range.