Objective: In cognitively normal adults, nutritional parameters are related to cognitive decline and incidence of dementia. Studies on the role of nutrition in predementia stages subjective cognitive decline and mild cognitive impairment, and mild stages of Alzheimer's disease (AD) dementia in a clinical setting are lacking. In the absence of a curative treatment, this evidence is important for targeting nutritional factors to potentially prevent or delay further cognitive decline. Our aim is to investigate associations of nutritional parameters with clinical progression in patients ranging from those who are cognitively normal to those who have AD dementia. Design: Longitudinal. Setting and Participants: Memory clinic, 551 patients (219 with subjective cognitive decline, 135 with mild cognitive impairment, and 197 with AD dementia), mean age 64 ± 8 years. Measurements: We assessed body mass index, fat-free mass, Mini-Nutritional Assessment, and dietary intake with the Dutch Healthy Diet food frequency questionnaire and the 238-item healthy life in an urban setting (HELIUS) food frequency questionnaire at baseline. Cox proportional hazard models were used to evaluate associations of nutritional parameters with clinical progression. Additional analyses were restricted to patients who were amyloid positive. Results: We observed clinical progression in 170 patients (31%) over 2.2 ± 0.9 years. Poorer Mini-Nutritional Assessment score [hazard ratio (95% confidence interval) 1.39 (1.18–1.64)], lower body mass index [1.15 (0.96–1.38)], lower fat-free mass [1.40 (0.93–2.10)], and a less healthy dietary pattern [1.22 (1.01–1.48)] were associated with a higher risk of clinical progression. Similar effect sizes were found in patients who were amyloid positive. Conclusions and Implications: Poorer nutritional status and a less healthy dietary pattern are associated with a higher risk of clinical progression. This study provides support for investigating whether improving nutritional status can alter the clinical trajectory of AD.