Spinocerebellar ataxia type 1 (SCA1), due to the expansion of a polyglutamine repeat within the ubiquitously expressed Ataxin-1 protein, leads to the premature degeneration of Purkinje cells (PCs), the cause of which is poorly understood. Here, we identified the unique proteomic signature of Sca1^154Q/2Q PCs at an early stage of disease, highlighting extensive alterations in proteins associated with synaptic functioning, maintenance, and transmission. Focusing on Homer-3, a PC-enriched scaffold protein regulating neuronal activity, revealed an early decline in its expression. Impaired climbing fiber-mediated synaptic transmission diminished mTORC1 signaling, paralleling Homer-3 reduction in Sca1^154Q/2Q PCs. Ablating mTORC1 within PCs or pharmacological inhibition of mTORC1 identified Homer-3 as its downstream target. mTORC1 knockout in Sca1^154Q/2Q PCs exacerbated and accelerated pathology. Reinstating Homer-3 expression in Sca1^154Q/2Q PCs attenuated cellular dysfunctions and improved motor deficits. Our work reveals that impaired mTORC1-Homer-3 activity underlies PC susceptibility in SCA1 and presents a promising therapeutic target.