Single-atom electrocatalysts (SAECs) have gained tremendous attention due to their unique active sites and strong metal-substrate interactions. However, current synthesis of SAECs mostly relies on costly precursors and rigid synthetic conditions, and often results in very low content of single-site metal atoms. Herein, we report an efficient synthesis method to prepare metal-nitrogen-carbon (M-NC) SAECs based on formamide condensation and carbonization, featuring a cost-effective general methodology for the mass production of SAECs with high loading of atomically dispersed metal sites. Seven types of single-metallic M-NC (Fe, Co, Ni, Mn, Zn, Mo and Ir), two bi-metallic (ZnFe and ZnCo) and one tri-metallic (ZnFeCo) SAECs were synthesized to demonstrate the generality of the methodology developed. Remarkably, these M-NC SAECs can be coated onto various supports with an ultrathin layer as pyrolysis-free electrocatalysts, among which the carbon nanotube-supported Fe-NC and Ni-NC SAECs showed high performance for O2 reduction reaction (ORR) and CO2 reduction reaction (CO2RR), respectively. Furthermore, the pyrolysis products of supported M-NC can still render isolated metallic sites with excellent activity, as exemplified by the bi-metallic FeCo-NC SAEC, which exhibited outstanding ORR performance in both alkaline and acid electrolytes by delivering ~70 and ~20 mV higher half-wave potentials than that of commercial 20 wt.% Pt/C, respectively. This work offers a feasible approach to design and manufacture SAECs with tuneable atomic metal components and high density of single-site metal loading, and thus may accelerate the deployment of SAECs for various energy technology applications.