Zinc sulfide, both as a bulk material and in nanocrystalline form, is a valuable luminescent material with important applications. Doped ZnS nanoparticles of around 5 nm are the material of choice for optoelectronic applications running in the UV region owing to their significant quantum size effect. This paper concerns detailed structural, spectroscopic and crystal field studies of ZnS nanoparticles, both pure and doped with Mn²⁺ ions, successfully synthesized at room temperature using a simple reverse micelle technique in the Triton X-100/cyclohexane medium. The resulting ZnS sphalerite phase small-size nanoparticles (3–5 nm) have a much larger energy band gap (~4.7 eV) than that reported for the bulk ZnS (3.6 eV), thus confirming a pronounced quantum confinement effect. The electron paramagnetic resonance data provided evidence for the existence of two distinct environments for Mn²⁺ ions: the interior (core) and near the surface of the nanoparticles. The presence of an Mn²⁺-characteristic orange emission centered at 600 nm confirmed that our samples were properly doped with Mn²⁺ ions, as the ⁴T₁→⁶A₁ radiation transition could arise only on the basis of Mn²⁺ ions incorporated into the ZnS nanoparticles. To the best of our knowledge, our finding include the longest decay time component for the orange emission ever observed, timed at about 3.3 ms. The experimental excitation spectra were analyzed and the transitions assigned using the exchange charge model of theory of crystal field, which allowed to calculate the energy level scheme of the Mn²⁺ ions. The results presented in this paper provide us with detailed information about the ZnS sphalerite nanocrystals studied and can be readily applied to other similar systems.