In this cohort study, positivity on quantitative tau PET, but not Aβ PET or MRI, provided a better prediction of conversion from MCI to all-cause dementia when added to a base model including age, sex, education, and Mini-Mental State Examination score, while prediction of Alzheimer disease (AD) dementia was improved with quantitative tau PET as well as tau PET visual reads. Here, MAPT is linked to early-onset autosomal dominant Alzheimer disease.