Post-thyroidectomy Horner’s syndrome (HS) is a rare occurrence, and its probability increases when a modified radical neck dissection is performed. We present a case of a patient with papillary thyroid carcinoma who presented with Horner’s syndrome one week after the right lateral dissection of the cervical lymph nodes. She underwent a complete thyroidectomy four months prior to this surgery. Both surgeries were uneventful intraoperatively. On examination, the right eye (RE) had partial ptosis with miosis and the absence of anhidrosis. A pharmacological test with phenylephrine 1% was used to localize the interruption of the oculosympathetic pathway with postganglionic third-order neuron involvement. She was treated conservatively, and her symptoms improved over time. Horner’s syndrome is a rare and benign complication of post-thyroidectomy surgery with radical neck dissection surgery. Since it does not compromise visual acuity, the disease is constantly overlooked. However, in view of the facial disfigurement and the possibility of incomplete recovery, the patient needs to be forewarned regarding this complication.