Horner syndrome (HS) is a rare complication of anterior cervical decompression and fusion procedures (ACDF). A 42-year-old female presented with sudden-onset weakness in both upper and lower limbs secondary to trauma and was diagnosed with a spinal cord injury with tetraplegia. Her pre-operative findings were that her motor level of injury was C4 on the right and C5 on the left, and her sensory level of injury was C4 and C5, respectively, on the right and left sides. Her neurological injury level (NLI) was C4, and her ASIA Impairment Scale score was A. The cervical spine MRI suggested compression fractures of the C5 and C6 vertebral bodies with cord compression. She underwent a C5 and C6 central corpectomy and mesh cage fusion by an anterior longitudinal incision via right-sided exposure. She developed ptosis, miosis, and anhidrosis on the side immediately after surgery. During rehabilitation admission, her neurological findings were that her motor level of injury was C4 on the right and C5 on the left, and her sensory level of injury was C4 and C5, respectively, on the right and left sides. Her NLI was C4, and her ASIA Impairment Scale score was C. Even one year after surgery, the symptoms persisted. HS is a rare complication of anterior cervical spine fixation, and it is essential to have a thorough understanding of intraoperative and postoperative ACDF-related complications to avoid them whenever possible and manage them successfully and safely when they occur.