Immune checkpoint inhibitors (ICPIs) are a breakthrough therapy in oncology and have been approved by the Food and Drug Administration for the treatment of several malignancies. ICPIs have been reported to cause immune-mediated damage of islet cells leading to ICPI-induced type 1 diabetes mellitus (T1DM). These reports described patients presenting with severe diabetic ketoacidosis (DKA). We present a case of a 69-year-old Caucasian male with type 2 diabetes suffering from non-small cell lung cancer and undergoing treatment with pembrolizumab, an anti-programmed cell death protein-1 antibody, who presented to our emergency department with complaints of nausea, vomiting, polyuria, and polydipsia. He was found to have high anion gap metabolic acidosis with ketosis and elevated blood glucose levels consistent with DKA. Lab workup was consistent with T1DM. Despite being on a tailored insulin regimen, his blood glucose remained elevated, necessitating the addition of metformin to his regimen which effectively controlled his blood glucose.