Nowadays, the inorganic cesium-based lead trihalide mixed perovskites quantum dots (QDs) are emerging as a promising candidate in display technology, light-emitting diodes, and solar cells. But theoretically, the properties of these mixed halide perovskites have not been investigated to a large extent for their novel application in various domains like optoelectronics and photonics. In this article, we have tuned the structural, photophysical, and electronic properties of CsPb(Cl/Br)3 by Mn doping, exploiting density functional theory (DFT) and lab experiments. Theoretically, after performing optimization in geometry, an increment in the lattice constant value has been observed by doping Mn in CsPb(Cl/Br)3 QDs from 5.592 to 5.624 A. These outputs are in accordance with the experimentally evaluated lattice constant values from X-ray diffraction peaks (5.580–5.605 A) and images of high-resolution transmission electron microscopy (5.581–5.629 A). Also, a decrement in the bandgap (3.105–2.943 eV) of Mn-doped CsPb(Cl/Br)3 QDs has been observed, which is consistent with the results obtained from the Tauc plot (3.1–2.9 eV). Redshift behavior has been observed from the absorption coefficient and dielectric spectrum due to the introduction of Mn as dopant impurity in the host CsPb(Cl/Br)3 QDs. This ab initio study with experimental investigation provides a new path to explore and tailor the various fascinating properties of these types of materials.