A 55-year-old man presented with left buttock and lower leg pain of 9 months' duration, initially misdiagnosed as a herniated lumbar disc. Lumbar MRI revealed a right L5 nerve root cyst, but symptoms worsened, examination revealed left paraspinal tenderness, grade IV left lower extremity weakness, hyperesthesia, and positive straight leg raise tests. Selective nerve root block localized the lesion to the left L5 root, contrast MRI identified an L4-5 facet joint cyst with inflammation. Uni-portal Spinal Surgery (AUSS) endoscopic resection with arthroscopic assistance confirmed the schwannoma histopathologically. After surgery, the pain on the numerical rating scale (NRS) decreased from 7 to 2 by 24 hours, discharge occurred on the 6th day. At the 2-month follow-up, the symptoms had completely resolved. The case highlights the diagnostic pitfalls of lumbar schwannoma mimicking disc herniation or cyst and the role of nerve root blocks, contrast-enhanced MRI, and histopathology.