A simple and low-energy-consuming approach to synthesize highly stable and dispersive silver nanoparticle–graphene (AgNP–GE) nanocomposites has been developed, in which the stability and dispersivity of the composites are varied greatly with the pH value and temperature of the reaction. The results demonstrate that the optimal reaction conditions are pH 11 at room temperature for 70 min. As-synthesized composites display excellent antimicrobial activity, and can completely inhibit the growth of Escherichia coli cells at a concentration of 20 mg L−1 (20 ppm). After treatment with 10 ppm AgNP–GE composites, the cells are killed completely within 3 h. The unique structure imparts such good antimicrobial properties to the composites. Firstly, the sheetlike AgNP–GE tends to be adsorbed and accumulated onto the surface of cells, which can change the permeability and enhance the antimicrobial activity. Secondly, Ag+ released from AgNPs can act on the cells effectively and fully, thereby resulting in cell death.