The dentate gyrus (DG) of the hippocampus is one of the few brain regions where new neurons are continuously generated throughout adulthood. Adult-born granule cells (ABGCs) in the DG are thought to support pattern separation, the process by which overlapping experiences are transformed into distinct memory traces. Although behavioral studies implicate ABGCs in this process, the specific computational mechanisms by which they modulate activity or plasticity in downstream hippocampal circuits remain unclear. Here we performed a modified spontaneous location recognition task in mice to assess how reversible silencing of ABGCs affects behavior, and to examine how their activity, as well as that of downstream circuits, contributes to behavior. We show that silencing even a small cohort of young ABGCs disrupts pattern separation, revealing that -during their critical maturation window-, these neurons exert a disproportionately strong influence on DG-dependent behavior. We also characterize CA1 spatial maps obtained through high-throughput calcium imaging microendoscopes. Collectively, these results link behavior and population imaging to reveal how adult neurogenesis shapes hippocampal circuits involved in spatial discrimination and memory.