Mucopolysaccharidosis type IIIA (MPS IIIA) is predominantly a disorder of the central nervous system, caused by a deficiency of sulfamidase (SGSH) with subsequent storage of heparan sulfate-derived oligosaccharides. No widely available therapy exists, and for this reason, a mouse model has been utilized to carry out a preclinical assessment of the benefit of intraparenchymal administration of a gene vector (AAVrh10-SGSH-IRES-SUMF1) into presymptomatic MPS IIIA mice. The outcome has been assessed with time, measuring primary and secondary storage material, neuroinflammation, and intracellular inclusions, all of which appear as the disease progresses. The vector resulted in predominantly ipsilateral distribution of SGSH, with substantially less detected in the contralateral hemisphere. Vector-derived SGSH enzyme improved heparan sulfate catabolism, reduced microglial activation, and, after a time delay, ameliorated GM3 ganglioside accumulation and halted ubiquitin-positive lesion formation in regions local to, or connected by projections to, the injection site. Improvements were not observed in regions of the brain distant from, or lacking connections with, the injection site. Intraparenchymal gene vector administration therefore has therapeutic potential provided that multiple brain regions are targeted with vector, in order to achieve widespread enzyme distribution and correction of disease pathology.