Manganese sulfide and Cu-doped manganese sulfide nanocrystals have been synthesized by wet chemical technique. The structural, optical, electrical and magnetic properties of as-synthesized nanocrystals have been investigated. The average crystallite size and lattice strain of the samples have been calculated from powder X-ray diffraction patterns using the Williamson–Hall analysis. The results show that the average crystallite size decreased while both the lattice strain and the dislocation density values increased in the Cu-doped MnS nanocrystals. The surface morphology of Cu-doped MnS nanocrystals has lesser particle size than undoped sample and it shows spherical like structures with little agglomeration. The chemical composition of the prepared samples has been obtained from EDAX. It clearly indicates the presence of Cu ions in the MnS lattice. UV–visible spectroscopy shows a blue shift in the optical band gap with doping. The photoluminescence spectra on the doped sample show a quenching of the PL intensity due to strain induced by doping. The electrical conduction, dielectric and impedance properties of as-synthesized nanocrystals have been investigated in the frequency range 50 Hz–5 MHz and temperature range 323–473 K which are greatly affected by doping with Cu. The vibrating sample magnetometer measurement revealed that the undoped MnS has paramagnetic behavior while the Cu-doped MnS has superparamagnetic behavior. On Cu-doping, the saturation magnetization and remanence increases while the coercivity decreases.