A method using glassy carbon electrode containing biochar, nanodiamonds and chitosan has been developed for simultaneous voltammetric quantification of cadmium and lead. The developed material was characterised by microscopy (transmission electron and scanning electron) and square-wave anodic stripping voltammetry. Using optimum conditions, the sensor exhibited excellent response for the simultaneous quantification of metal ions. High electrochemical sensitivities of 0.42 and 5.3 μA μmol−1 cm−2 was found for Cd and Pb quantification, respectively. Well separated peaks at −0.64 and −0.42 V vs. Ag/AgCl(KClsat) were obtained, as well as low limits of detection of 0.11 and 0.056 µmol L−1 and linear ranges of 1.0 to 75.0 and 0.25 to 6.0 μmol L−1 for cadmium and lead quantification, respectively. The modified GCE exhibited good repeatability, stability for 30 days and reproducibility (with a relative standard deviation between 4.6% and 4.9% for n = 5). The developed sensor presents high robustness, requiring no step cleaning of the electrode surface to remove any remaining bonded metals. An ecologically friendly, low cost and high yield electrode was therefore developed for the simultaneous determination of two toxic metals in real environmental samples applying square-wave anodic stripping voltammetry.