Treatment of refractory, unresectable cutaneous squamous cell carcinoma presents a great challenge in head and neck oncology with poor prognosis. Prior case reports have shown off-label pembrolizumab, a programed cell death receptor antagonist, can be effective in unresectable cutaneous squamous cell carcinoma. Furthermore, prior reports have suggested enhanced efficacy when high mutational burden is present. In this study we present a severe case of unresectable cutaneous squamous cell carcinoma invading the orbit and cavernous sinus with documented tumor MLH1 mutation. The patient had a complete response to palliative, off-label pembrolizumab therapy.