Acute immune-mediated demyelinating disease is an acute immune-mediated inflammatory demyelinating disorder of the central nervous system that predominantly affects children and young adults. Spinal cord involvement may occur in some cases and can contribute to significant neurological deficits. We report a rare case of multifocal demyelination involving both the brain and spinal cord in a pediatric patient presenting with acute onset fever, altered sensorium, seizures, limb weakness, and impaired motor function following a preceding febrile illness. Magnetic resonance imaging revealed multifocal hyperintense demyelinating lesions involving the cerebral white matter along with extensive spinal cord involvement suggestive of an acute inflammatory demyelinating process. Cerebrospinal fluid and laboratory investigations aided in excluding infectious and other autoimmune etiologies. The patient was managed with corticosteroid therapy and supportive care, resulting in gradual clinical improvement. This case highlights the importance of early recognition of acute demyelinating disorders with spinal involvement, timely neuroimaging, and prompt initiation of immunomodulatory therapy to reduce neurological morbidity and improve clinical outcomes.