Formamidinium lead iodide (FAPbI3) has gained considerable interest as a promising photoactive layer for optoelectronic devices due to its broad spectrum of light absorption and increased thermal stability when compared to its conventional methylammonium counterpart (MAPbI3). Recent developments in substituting formamidinium (FA) with smaller Cs cations have further accelerated its growth in the photovoltaic (PV) community by enhancing phase stability and power conversion efficiency (PCE). However, only a few studies are reported on perovskite photodetectors (PPDs). Here, the influence of Cs incorporation is investigated on PD performance in the CsXFA1-XPbI3 system (X = 0–0.25). Finding that the partial substitution of FA with Cs cations alleviates lattice strain by increasing crystallinity and inducing a well-ordered surface morphology with vertical crystallographic orientation, which suppresses non-radiative recombination mechanisms. Such improved physicochemical properties of the mixed-cation perovskite light absorbers can improve the PD performance by reducing the dark and noise current to values as low as 3.3 × 10−9 A cm−2 and 6.1 × 1011 A Hz−1/2, thereby enabling PPDs with a faster photoresponse and greater sensitivity, which holds great promise for future optoelectronics applications.