Nanocomposites from nitrogen-doped graphene (NG) and magnetic nanoparticles (MNPs) are prepared via an edge-functionalized ball milling approach, followed by a coprecipitation method. Catalytic peroxidase-like activity of the resulting MNP/NG is investigated. The as-synthesized MNP/NG nanocomposites are used to catalyze the colorimetric reaction between hydrogen peroxide (H2O2) and 2,2′-azino-bis(3-ethylbenzo-thiazoline-6-sulfonic acid) diammonium salt (ABTS). Typical absorption peaks associated with the oxidation of ABTS are found to increase linearly with increasing H2O2 concentrations up to 10.0 × 10−3 m with a detection limit of 17.1 × 10−6 m. Rapid detection of H2O2 with the use of MNP/NG is confirmed further by a glucose biosensor. Concentrations of glucose could be monitored by detection of H2O2 since H2O2 is generated during oxidation of glucose by glucose oxidase (GOx). The absorption intensity increases significantly with increasing glucose amounts up to 18.0 × 10−3 m glucose, with a detection limit of 59.7 × 10−6 m, indicating a high performance for glucose detection.