Two-dimensional (2D) halide perovskites are promising materials for environmen-1 A c c e p t e d m a n u s c r i p t tally stable next-generation optoelectronic device applications. Besides the widely investigated Ruddlesden-Popper (RP) phases, Dion-Jacobson (DJ) phases are attracting considerable attention due to their rapid emergence as efficient solar cell materials. However, there is very little atomistic understanding of the charge carrier dynamics at ambient conditions for these DJ-phases, limiting the possibilities to tune their optoelectronic performances through compositional engineering routes. Here, by combining nonadiabatic molecular dynamics with time-domain density functional theory methods at room temperature, we compare the dominant non-radiative carrier recombination and dephasing processes in RP and DJ monolayered lead halide perovskites. Our systematic study demonstrates that performance-limiting nonradiative carrier recombination processes greatly depends on the electron-phonon interactions induced by structural fluctuations, and instantaneous charge localization in these materials. The stiffer interlayer packing due to the presence of single spacer dications, which separates the lead iodide slabs, reduces the thermal fluctuations in the DJ-phase to a greater extent than that in the RP-phase 2D-perovskites. Specific electronic coupling between the closely spaced lead iodide layers enhances delocalization of band-edge charge densities in DJ-phase systems. Compared to the RP-phase, reduced inelastic electron-phonon scattering in DJ-phase perovskites significantly limits intrinsic non-radiative recombination processes. Consequent enhancement in photogenerated charge carrier lifetime makes DJ-phase perovskites potentially suitable for various optoelectronic devices. The computational insights gained from this study allow us to outline a set of robust design principles for DJ-phase perovskites to strategically tune their optoelectronic properties.