The speed and severity of clinical progression after Alzheimer disease (AD) diagnosis varies and depends on multiple factors, most not well elucidated. We assessed whether body mass index (BMI) and 1-year weight change (WC) are associated with clinical progression in amnestic mild cognitive impairment (aMCI) and early-stage AD. Longitudinal data comprising 2268 aMCI and 1506 AD participants in the National Alzheimer's Coordinating Center's Uniform Data Set were used to examine nuances of clinical progression by BMI and WC, as well as potential variations in associations by age, sex, BMI (WC model), or apolipoprotein E genotype. In aMCI, high BMI (vs. moderate BMI) was associated with slower progression; weight loss (vs. no WC) was associated with faster progression. In AD, no significant differences were observed in clinical progression by BMI or WC. The association between BMI and clinical progression varied significantly by apolipoprotein E genotype in AD, and the association between WC and clinical progression varied significantly by sex and BMI in aMCI. Baseline BMI and 1-year WC in late life may serve as early prognostic indicators in aMCI and early-stage AD. If replicated, these results may help in counseling patients on anticipated clinical progression and suggest windows of opportunity for intervention.