CRISPR-mediated gene activation (CRISPRa) is a promising therapeutic gene editing strategy without inducing DNA double-strand breaks (DSBs). However, in vivo implementation of these CRISPRa systems remains a challenge. Here, we report a compact and robust miniCas9 activator (termed miniCAFE) for in vivo activation of endogenous target genes. The system relies on recruitment of an engineered minimal nuclease-null Cas9 from Campylobacter jejuni and potent transcriptional activators to a target locus by a single guide RNA. It enables robust gene activation in human cells even with a single DNA copy and is able to promote lifespan of Caenorhabditis elegans through activation of longevity-regulating genes. As proof-of-concept, delivered within an all-in-one adeno-associated virus (AAV), miniCAFE can activate Fgf21 expression in the liver and regulate energy metabolism in adult mice. Thus, miniCAFE holds great therapeutic potential against human diseases.