Introduction Mapping the preclinical dementia phase is important for early detection and evaluation of interventions. We assessed the trajectories of cognitive decline in preclinical dementia over 12 years and investigated whether being a fast decliner across 6 years is associated with increased risk of dementia the following 6 years. Methods Rates of cognitive decline were determined using mixed‐effects models for 1646 participants from the Swedish National Study on Aging and Care in Kungsholmen (SNAC‐K) cohort. Cox regression was used to assess the future likelihood of dementia for fast decliners (declining ≥1.5 standard deviations SDs faster than the age‐specific rates). Results Participants in a preclinical phase of dementia showed increased rates of decline in all cognitive tests compared to the no‐dementia group, particularly closer (0‐6 years) to diagnosis. Participants declining fast in three or more cognitive tests 12‐6 years before diagnosis demonstrated a high risk of dementia 6 years later (hazard ratio HR 3.90, 95% confidence interval CI 2.28–6.69). Discussion Being a fast decliner is linked to increased risk of future dementia.