Nanomaterial-based enzyme mimics (nanozymes) are attracting increasing attention because of their low production cost, high stability against denaturation, and resistance to high concentrations of substrates. Here, carbon nanoparticles doped with a small amount (<5 mol%) of Pt (denoted as PtCNPs) are synthesized via a facile, cost-effective hydrothermal treatment of p-phenylenediamine (PPD) and KPtCl. The obtained PtCNPs possess high aqueous stability, excellent water-dispersibility, and suitable size (∼15 nm). More interestingly, the PtCNPs exhibit an intrinsic peroxidase-like activity that can quickly catalyze 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (HO) and produce a blue color. Importantly, since satisfactory catalytic properties were also observed when KPtCl was replaced with CuCl, NiCl, or NaPdCl during the synthesis, the PPD- and inorganic metal salt-involved hydrothermal synthetic approach may be developed as a general and simple way to fabricate new nanozymes. Besides, the steady-state kinetics reveals that the PtCNPs have a stronger affinity for TMB and a weaker affinity for HO compared with horseradish peroxidase. On the basis of the color reaction, a colorimetric detection method for HO and glucose has been successfully established with a detection limit of 0.15 and 0.30 μM, respectively. Further, the method has also been successfully applied for glucose detection in human serum samples. To sum up, this work develops a new synthetic method of metal-doped carbon nanomaterials and demonstrates their capability for the sensitive and selective detection of HO and glucose, which may foster the development of new nanozymes for biosensing applications.