Lipin-1 () is essential in lipid metabolism, with mutations commonly causing severe, recurrent rhabdomyolysis, especially in children. Here, we present a rare case of adult-onset myopathy and rhabdomyolysis in a 48-year-old male firefighter, a heterozygous carrier of an exon 18 deletion. The patient experienced fatigue, muscle loss, and exercise intolerance over two to three years. Initial evaluations revealed mildly elevated creatine kinase (CK) levels (297 U/L), with unremarkable results from further tests, including electromyography, antinuclear antibodies, myasthenia gravis antibody, and myositis antibody panels. Six months later, he had worsening muscle stiffness, pain, and darkened urine. In the emergency department, his CK was 4000 U/L, with elevated aldolase and transaminases. Hospitalization and hydration treatment normalized his CK levels. Genetic testing through a Comprehensive Neuromuscular Disorder Panel identified a heterozygous exon 18 deletion. To our knowledge, this case is significant as the first reported instance of adult-onset myopathy and rhabdomyolysis in a heterozygous carrier without statin exposure. While mutations typically cause pediatric-onset rhabdomyolysis, this case highlights the need to consider mutations in adults with episodic myopathy and rhabdomyolysis when other causes are ruled out. Genetic testing is crucial for diagnosis and management.