Metal halide perovskite nanoplatelets (NPls) exhibit attractive optical properties as a result of quantum and dielectric confinement effects. Their strong and narrow photoluminescence (PL), tunable over the entire visible spectral range, make them highly promising for light-emission and lasing applications. In metal halide perovskites, partial cation substitution serves as an additional tool for fine-tuning their structure and optical properties. While A-site cation substitution has been successfully used for perovskite nanocrystals, its application to their NPls counterparts was not widely addressed yet. In this study, we demonstrate that sufficient surface passivation is required to be performed prior to A-site cation exchange in CsPbI3 perovskite NPls by treatment with ZnI2 and oleylamine, enabling efficient A-site cation substitution with both smaller (Rubidium, Rb+) and larger (formamidinium, FA+) cations. This allowed us to tune the PL peak position of NPls over broad spectral range of 590 to 630 nm. In particular, mixed-cation Cs1-xFAxPbI3 (x=0.68−0.87) NPls exhibited narrow (full width at half maximum ~25 nm) and efficient (PL quantum yield ~ 90%) pure-red emission at 620-626 nm, making them promising candidates for light emission applications. We have furthermore demonstrated that the partial A-site substitution of (Cs+) towards organic (FA+) cations enhanced nonlinear optical responses of Cs1−xFAxPbI3 NPls, whose TPA cross-section increased up to 3.85×106 GM, positioning them as a promising material for nonlinear optical applications.