Development of nontoxic quantum dots (QDs) exhibiting superior photoluminescence properties is a major challenge in the commercialization of QDs-based solid-state lighting or display device applications. This work reports the synthesis of heavy-metal-free Ag:Zn−In−S/ZnS (Ag:ZIS/ZS) core/shell QDs via the heating-up method followed by a nonaqueous hot-injection route. Formation of the QDs, their growth, and structural qualities were studied by high-resolution transmission electron microscopy and XRD analysis. The optical properties of these QDs were investigated through absorption, photoluminescence, and time-resolved fluorescence decay spectroscopic measurements. Surface passivation of the core Ag:ZIS QDs was done by coating ZnS layers on them. The successful doping of Ag + into the ZnInS host material changed the emission characteristics; the influence of Ag loading and the shell thickness on the optical features were studied. The shell thickness had a strong impact on the photophysics of the nanocrystal with enhanced radiative lifetime. By regulating the Ag concentration, the Ag:ZIS core QDs showed a wide ranging tunable emission from 392 to 547 nm with a prominent blue shift with the introduction of ZnS shell layers. The blue shift in the band gap energy and emission wavelength has been reported to be due to the cation-exchange process of Ag:ZIS/ZS core/shell QDs. The radiative excited state lifetime is prolonged with subsequent injection of shells with the highest reported value being 504.72 ns. These nontoxic core/shell QDs, endowed with notable spectral emission within the visible spectrum, can be used as a down-converting material by tailoring the dopant concentration and shell precursor injection. Furthermore, a nanocomposite film of PMMA−QDs was synthesized by the drop-casting method, and its overall luminous efficacy complements that of parental core/shell QDs without quenching. The transmittance plot delineates that the optical transparency of the film can be engineered over the entire visible spectrum by varying the dopant (Ag) concentration.