Exploring non-noble metal dual-functional electrocatalysts with high activity and stability for water splitting is highly desirable. In this study, using zeolitic imidazolate framework-L (ZIF-L) nanoarrays as the precursor, manganese oxide-decorated porous nickel–cobalt phosphide nanosheet arrays have been prepared on nickel foam (denoted as MnO_x/NiCoP/NF) through cation etching, phosphorization and electrodeposition, which are utilized as an efficient dual-functional electrocatalyst for overall water splitting. The hierarchical porous nanosheet arrays provide abundant active sites for the electrochemical process, while the MnO_x modification induces strong interfacial interaction, benefiting charge transfer. Thus, the MnO_x/NiCoP/NF exhibits excellent electrocatalytic activity toward the hydrogen evolution reaction (HER, overpotential of 93 mV at 10 mA cm⁻²), oxygen evolution reaction (OER, overpotential of 240 mV at 10 mA cm⁻²) and overall water splitting (cell voltage of 1.59 V at 10 mA cm⁻²). Furthermore, it shows superior stability during continuous overall water splitting for 200 h. This work provides a simple and effective approach for developing efficient non-noble metal dual-functional catalysts for overall water splitting.