Organometal halide perovskites are being extensively studied as they can serve as an excellent active medium in various optoelectronic devices such as solar cells, light-emitting diodes (LEDs), lasers, etc. We report the fabrication of highly controlled single-phase mixed halide two-dimensional (2D) perovskites by successive doping of inorganic dopant, potassium iodide (KI), in 2D perovskite 2-(1-cyclohexenyl) ethylammonium lead bromide, (C6H9C2H4NH3)2PbBr4 (CHPB). Our computational investigations further confirm the thermodynamic stability of mixed anion lattices. A stoichiometric increase of KI in CHPB beyond critical passivation levels results in a uniform bandgap tunability of thin films from the UV (∼3.21 eV) to green (∼2.50 eV) region of spectra. Distinctly linear, tunable, and single-phase strong room-temperature exciton absorbance (∼401−508 nm) and emission peaks (∼416−518 nm) in the blue-green spectral region are observed with the increase of KI concentration levels in thin-film samples. Doped two-dimensional (2D) perovskite films exhibit a minimum stokes shift parameter of 40 meV, which is very small compared to the conventional route of mixing two perovskite precursor solutions. X-ray diffraction studies show that the layered structure of doped 2D perovskite thin films remains intact despite high KI concentration levels. Charge transport studies of doped 2D perovskite thin films demonstrate a decline in the lifetime of photogenerated charge carriers with an increase of iodide concentration. Combining several experimental and computational techniques, we find that increased carrier effective masses and consequent formation of strongly bound excitons cause the decrease in carrier lifetime. In short, our present study reveals a promising low-cost solution-processable approach to fabricate single-phase mixed halide 2D perovskites to achieve unprecedented exciton tunability in low-dimensional optoelectronic materials. © 2020 American Chemical Society