The add-on heat sink includes an elongate base having a plurality of fins extending from a surface thereof. A magnetic layer is disposed on the bottom of the base, which permits the add-on heat sink to be installed on any ferromagnetic heated surface. The magnetic layer is composed of either a polymer matrix having a plurality of thermally conductive structural components and a plurality of magnetic particles dispersed therein, or a thermally conductive polymer having magnetic particles dispersed therein. Alternatively, if the heated surface is not ferromagnetic, the heat sink may be magnetically attached by adhesively attaching mating magnetic and ferromagnetic pads to the heat sink and to the heated surface. This configuration allows the add-on heat sink to be installed with minimal footprint. Optionally, a fan may be magnetically attached to the heat sink to cool the heated surface by both conduction and convection.