Cooling of electronic components is important as demand for reduced component size and the increased heat generation rate cause the heat flux to increase. Liquid-cooled heat sinks work better than air-cooled heat sinks due to the improved heat transfer capability of liquids over air. If the thermal properties of the liquids are further improved, the performance of the heat sinks can be increased in terms of their capacity of heat removal at reduced size. In this paper, the thermal properties of water are altered by adding Al2O3-Cu nanocomposite powder. This nanocomposite powder is synthesized in a thermochemical route followed by a hydrogen reduction technique. The synthesized nanocomposite powder is characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. Al2O3-Cu/water hybrid nanofluid is prepared and tested in a thin-channeled copper heat sink of overall dimensions 59 × 59 × 12.6 mm. The effect of hybrid nanofluid in the enhancement of convective heat transfer and pressure drop is studied. The experimental results demonstrate that the convective heat transfer coefficient of the heat sink is increased significantly when hybrid nanofluid is used as the working fluid compared with water. The rise in pumping power with the use of hybrid nanofluid compared with water is less than the rise in the convective heat transfer coefficient.