Humans and mice with mutations in COL4A1 and COL4A2 manifest hallmarks of cerebral small vessel disease (cSVD). Mice with a missense mutation in Col4a1 at amino acid 1344 (Col4a1^+/G1344D) exhibit age-dependent intracerebral hemorrhages (ICHs) and brain lesions. Here, we report that this pathology was associated with the loss of myogenic vasoconstriction, an intrinsic vascular response essential for the autoregulation of cerebral blood flow. Electrophysiological analyses showed that the loss of myogenic constriction resulted from blunted pressure-induced smooth muscle cell (SMC) membrane depolarization. Furthermore, we found that dysregulation of membrane potential was associated with impaired Ca²⁺-dependent activation of large-conductance Ca²⁺-activated K⁺ (BK) and transient receptor potential melastatin 4 (TRPM4) cation channels linked to disruptions in sarcoplasmic reticulum (SR) Ca²⁺ signaling. Col4a1 mutations impair protein folding, which can cause SR stress. Treating Col4a1^+/G1344D mice with 4-phenylbutyrate, a compound that promotes the trafficking of misfolded proteins and alleviates SR stress, restored SR Ca²⁺ signaling, maintained BK and TRPM4 channel activity, prevented loss of myogenic tone, and reduced ICHs. We conclude that alterations in SR Ca²⁺ handling that impair ion channel activity result in dysregulation of SMC membrane potential and loss of myogenic tone and contribute to age-related cSVD in Col4a1^+/G1344D mice.