Pathological amyloid beta (Aβ) accumulation in the brain has been associated with poor cognitive performance and aberrant neuronal activity in preclinical mouse models of Alzheimer’s disease (AD). Here, we assessed the impact of advanced Aβ pathology on spatial information processing in the medial entorhinal cortex (MEC) of 18-month APP NL-G-F knock-in mice as they explored contextually novel and familiar open field arenas in a two-day, four-session recording paradigm. Using in vivo electrophysiology, we tracked single-units across all sessions and found that MEC single-unit spatial information scores were decreased in APP-KI mice versus age-matched C57BL/6J controls. MEC spatial representations were also impacted in APP-KI mice. Border cell firing preferences were unstable across sessions and spatial periodicity in putative grid cells was disrupted. In contrast, MEC border cells and grid cells in Control mice were intact and stable across sessions. To assess the stability of spatial maps across sessions, we utilized metrics based on the Earth Mover’s Distance (EMD). Examining the EMD quantiles from APP-KI and Control single-units revealed an increased likelihood of instability in spike density EMD (Odds Ratio, APP-KI over Control units). Additionally, spatial decoding analysis of MEC single-units revealed deficits in position and speed coding generalization across sessions in APP-KI mice. Finally, MEC single-unit analysis revealed a weak hyperactive phenotype in APP-KI mice that appeared to be driven by narrow-spiking units (putative interneurons). These findings tie Aβ-associated dysregulation in neuronal firing to disruptions in spatial information processing that may underlie cognitive deficits previously reported in these mice.