Sandhoff and Tay-Sachs diseases are autosomal recessive lysosomal storage diseases caused by the deficiency in β-hexosaminidase A (HEXA), an enzyme required to catabolize GM2 ganglioside. HEXA is composed of α and β subunits, encoded by HEXA and HEXB genes, respectively. Mutations in HEXA gene lead to Tays–Sachs disease, and in HEXB, to Sandhoff disease. Patients affected by Sandhoff or Tay-Sachs diseases show severe, progressive neurodegeneration with clinically indistinguishable devastating pathology, and to date, there is no cure. Using Sandhoff and Tay-Sachs disease mouse models, we demonstrated therapeutic efficacy of a novel gene therapy approach based on a single intra-CSF administration of new bicistronic AAV9 vectors encoding simultaneously for both HexA and HexB genes. After treatment, Tay-Sachs and Sandhoff mice correctly expressed HEXA enzyme in the whole CNS. Moreover, AAV-treated Sandhoff mice showed reversion of primary GM2 and secondary (cholesterol) storage and normalitzation of lysosomal pathology in CNS, leading to correction of myelinization, autophagy and neuroinflammation. After AAV delivery, liver was efficiently transduced and allowed to secrete HEXA into circulation and correct peripheral GAG and cholesterol storage and lysosomal pathology of Sandhoff disease. This was paralleled to normalization of behavioral deficits, such as locomotor alterations, coordination, and mobility. Furthermore, this treatment markedly extended lifespan, compared with the co-administration of AAV-HexA and AAV-HexB monocistronic vectors. Altogether, our results demonstrated that a single intra-CSF administration of AAV-HexA-HexB bicistronic vectors led to whole-body correction of both CNS and peripheral Sandhoff disease, laying the foundation for the clinical translation to treat human patients.