Dementia is a major public health problem with substantial personal, social, and financial burden, affecting more than 47 million people worldwide, with no cure to date. The major types of dementia include Alzheimer’s disease (AD), Lewy Body dementia (LBD), and frontotemporal dementia (FTD), which show distinct and overlapping pathological, neurological, and cellular signatures, but their detailed molecular signatures remain uncharacterized. Here, we systematically profile the molecular signatures of AD, LBD, FTD, and healthy aging, at the single-cell level, across traits, individuals, brain regions, cell types, age, sex, and disease severity. We use genetic, epigenomic, and transcriptional profiles, generating a total of ~1.5 million genome-wide maps at the single-cell (sc) level using scRNA-seq and scATAC-seq across 768 post-mortem brain samples from the Religious Order Study and Memory and Aging Project (ROS MAP) cohorts. We analyze the resulting datasets in the context of genetic variation from whole-genome sequencing, and phenotypic variation from rich longitudinal profiling and cognitive evaluations, enabling us to discover genes, control regions, pathways, cell types, and brain regions playing causal roles in AD and ADRD, and how they vary across age, sex, and traits. The resulting datasets will help guide the search for new therapeutics, by providing detailed therapeutic targets, and the specific conditions where they are predicted to act.