Hydrocephalus is classically considered to be a disorder of altered cerebrospinal fluid (CSF) circulation, leading to the dilation of cerebral ventricles. Here, we report a clinical case of a patient who presented with fetal-onset hydrocephalus with diffusely reduced cortical and white matter volumes resulting from a genetic mutation in L1CAM, a well-known hydrocephalus disease gene involved in neuronal cell adhesion and axon development. After CSF was drained from the ventricle intraoperatively, the patient's cortical mantle collapsed and exhibited a "floppy" appearance on neuroimaging, suggesting an inability of the hydrocephalic brain to maintain its structural integrity. The case provides clinical support for altered brain biomechanical properties in human hydrocephalus and adds to the emerging hypothesis that altered brain development with secondary impact on brain structural stability may contribute to ventricular enlargement in some subsets of hydrocephalus.