A 6 year old female spayed Australian Shepherd dog presented to the Cornell Hospital for Animals’ Neurology Service for a 5 week history of neck and left forelimb pain and left-sided Horner's Syndrome. A thorough work-up lead to the definitive diagnosis of high grade soft tissue sarcoma ventral to the first, second, and third thoracic vertebrae. The patient was treated with palliative radiation therapy and oral chemotherapy. As of 5 months following this treatment, the patient is still alive and maintains a good quality of life. Horner’s syndrome is not an uncommon finding in dogs and cats. Brought about by a plethora of causes, the condition is only rarely associated with serious, life-threatening disorders. Every effort should be made to find a definitive cause in order to guide treatment and determine patient prognosis.