NCO/N, S-rGO is successfully synthesized for multifunctional applications, and the mechanisms of HMIs detection and HER are further explained. Display omitted •NiCo2O4 decorated N, S codoped reduced graphene oxides are synthesized.•The synergetic effect, oxygen vacancies and hybridization of NCO/N, S-rGO improve its excellent electrocatalytic performance.•NCO/N, S-rGO has multifunctional applications for the electrochemical detection of HMIs and HER activity. It is anenormous challenge to explore multifunctional electrocatalysts for simultaneous detection of Cd(II), Cu(II), Hg(II) and hydrogen evolution reaction (HER). In this work, NiCo2O4 decoratedN,Sco-doped reduced grapheneoxide composites (NCO/N, S-rGO) are synthesized via hydrothermal followed calcination process. The synergetic effect, oxygen vacancies and hybridization of NCO/N, S-rGO improve its excellent electrocatalytic performance for simultaneous and individual detection of Cd(II), Cu(II), Hg(II) and HER activity. When NCO/N, S-rGO is acted as heavy metal ions (HMIs) electrochemical sensors, it exhibits the high sensitivity and low detection limit for Cd(II) with 2.38 μA μM−1 and 123 nM, Cu(II) with 10.90 μA μM−1 and 14.4 nM, as well as Hg(II) with 5.41 μA μM−1 and 67 nM. NCO/N, S-rGO has outstanding stability and anti-interference performance and has been successfully implemented in the actual water environment. Furthermore, NCO/N, S-rGO also exhibits HER activity with overpotential and Tafel slope of 107 mV and 71.35 mV dec-1, respectively. The electrochemical detection and HER activity mechanism of NCO/N, S-rGO are further investigated and demonstrated that the synergetic effect of NCO/N, S-rGO can improve its electronic microstructure and promote electrocatalytic performance. This work not only provides a simple method for the preparation of multifunctional electrocatalysts, but also enhances the comprehension of HMIs detection and HER mechanism.