Introduction: Prolonged grief disorder (PGD) is a condition marked by sustained separation distress. Although cognitive models suggest that memory processes are critical in PGD, the specific characteristics of loss-related memories have received limited longitudinal attention. This study examined how loss-related memory characteristics and prolonged grief symptoms (PGS) influence each other over the first 18 months of bereavement. Method: A community sample of 275 bereaved adults (79% women, Mage = 46.4) completed assessments of PGS and the Oxford Grief–Memory Characteristics Scale at three timepoints (0–6, 6–12, and 12–18 months post-loss). Panel graphical vector autoregressive (GVAR) models decomposed contemporaneous and temporal with-in person associations over time. Results: Memory characteristics emerged as the most influential variables over time. Memories high in re-experiencing qualities predicted increasing difficulty accepting the death and a greater range of memory triggers over time, which predicted subsequent difficulty moving forward. Predominantly negative memories were associated with subsequent heightened yearning, while visceral consequences of memories predicted later loneliness, and disconnection from memories of the past self predicted role confusion. Re-experiencing qualities and the PGS emotional numbness had a bidirectional link. Contemporaneous networks showed dense connections among memory characteristics and symptoms within the same time point. Conclusion: Overall, the findings demonstrate that loss-related memory processes are predictors of later change in PGS during the first 18-month post-bereavement. This highlights maladaptive memory characteristics as promising mechanistic targets for intervention and supports therapeutic approaches that directly address how memories of loss are encoded, retrieved, and interpreted.