Pembrolizumab is a humanized monoclonal antibody that targets the programmed cell death 1 protein (PD-1) receptor and blocks the inhibitory checkpoint interaction between PD-1 and its ligands. This interaction leads to the upregulation of effector T-cells and downregulating regulatory T-cell production. Although this mechanism is essential for the management of cancer, it may lead to decreased self-tolerance with an autoimmune reaction toward healthy functioning tissue. One of the less commonly reported and less understood immune-related adverse events includes neuromuscular complications. We present a rare case of autoimmune demyelinating polyneuropathy and hydrocephalus secondary to pembrolizumab use for cutaneous squamous cell carcinoma of the cheek.