Signs and symptoms of Marcus Gunn phenomenom (MGP) may include the following:

• Mild-to-moderate drooping of the eyelids (blepharoptosis), usually on one side (unilateral)
• Synkinetic (involuntary) upper eyelid movement after almost any movement of the lower jaw including: mouth opening, chewing, sucking, jaw protrusion, clenching teeth together, and swallowing
• Worsening of symptoms when looking down
• Decreased vision due to amblyopia ("lazy eye")
• Strabismus ("crossed eyes")

These symptoms are usually first noticed in infancy while bottle or breastfeeding.

While it may not be needed in every case, people with Marcus Gunn phenomenon may benefit from surgery. Children with Marcus Gunn phenomenon need to be regularly monitored for amblyopia. If not addressed by age 7 to 10 years of age, amblyopia may become untreatable. Untreated severe ptosis can also result in vision loss. Other associated eye problems, such as horizontal strabismus, vertical strabismus, and double elevator palsy typically need to be addressed before considering how to manage Marcus Gunn phenomenon itself. These related eye problems may be corrected with eyeglasses, surgery, and/or medications.

Surgery for Marcus Gunn phenomenon may be considered for ptosis or jaw winking that is considered cosmetically significant, or causing amblyopia. Depending on the degree of ptosis and severity of jaw winking, several surgical techniques have been proposed. For mild Marcus Gunn phenomenon, management has included observation, levator muscle resection, and the Fasanella-Servat procedure.