Introduction Acute hyperammonemia in adult patients without liver disease is uncommon. Other etiologies are broad and can be difficult to rule out quickly. Urea cycle disorders such as ornithine transcarbamylase (OTC) deficiency are rare and often present in children. Adult-onset cases are rarer. This makes recognizing and treating late-onset urea cycle disorders challenging in the acute setting. Case Report We present the case of a 56-year-old male with a history of early onset coronary artery disease, chronic back pain, and heavy tobacco use who was found unconscious at home by a neighbor. He was found to have toxic levels of serum ammonia during further evaluation in the emergency department. Laboratory testing confirmed a diagnosis of OTC deficiency. Genetic analysis further showed a c.-106C>A variant of the OTC gene. Conclusion Although extremely rare, late-onset urea cycle disorders should be considered when patients present with altered mental status with hyperammonemia of unknown etiology. Furthermore, this case adds evidence that the c.-106C>A OTC gene variant has pathogenic significance.