A 4-year-old female spayed dog presented to the emergency department for non-ambulatory tetraparesis, which progressed to tetraplegia. Computed tomography (CT) confirmed cervical intervertebral disk extrusion at C5-6 extending to C6-7, and an emergency ventral slot was performed. After the procedure, the patient was placed on mechanical ventilation due to respiratory failure. Repeat assessment upon weaning her ventilatory support suggested the patient's neurological status had declined. Based on her deterioration and suspicion of progressive myelomalacia on magnetic resonance imaging (MRI), she was euthanized. Post-mortem histopathology of the spinal cord supported the presence of progressive myelomalacia. To the author's knowledge, this is the first case report describing a progressive myelomalacia in a canine patient with cervical intervertebral disk extrusion.