Heavy metals, notably Pb²⁺ and Cu²⁺, are some of the most persistent contaminants found in groundwater. Frequent monitoring of these metals, which relies on efficient, sensitive, cost-effective, and reliable methods, is a necessity. We present a nanocomposite-based miniaturized electrode for the concurrent measurement of Pb²⁺ and Cu²⁺ by exploiting the electroanalytical technique of square wave voltammetry. We also propose a facile in situ hydrothermal calcination method to directly grow binder-free mesoporous NiO on a three-dimensional nickel foam, which is then electrochemically seeded with gold nanoparticles (AuNPs). The meticulous design of a low-barrier Ohmic contact between mesoporous NiO and AuNPs facilitates target-mediated nanochannel-confined electron transfer within mesoporous NiO. As a result, the heavy metals Pb²⁺ (0.020 mg·L⁻¹ detection limit; 2.0-16.0 mg·L⁻¹ detection range) and Cu²⁺ (0.013 mg·L⁻¹ detection limit; 0.4-12.8 mg·L⁻¹ detection range) can be detected simultaneously with high precision. Furthermore, other heavy metal ions and common interfering ions found in groundwater showed negligible impacts on the electrode’s performance, and the recovery rate of groundwater samples varied between 96.3% ± 2.1% and 109.4% ± 0.6%. The compactness, flexible shape, low power consumption, and ability to remotely operate our electrode pave the way for onsite detection of heavy metals in groundwater, thereby demonstrating the potential to revolutionize the field of environmental monitoring.