The mental nerves, on each side of the chin, are the final branches of the lower part of the trigeminal nerve, and carry sensation from the chin. Since they're at the end of a very long route, diseases that affect the nerve anywhere along its path can result in numbness of the chin. Though seemingly a minor problem, chin numbness can point to serious medical problems and should always be evaluated by a doctor.
Loss of sensation in the chin area can be the first symptom of cancer. Chin numbness is often a bad prognostic sign and indicates progression of the underlying disease. A 2000 article in "Critical Review of Oncology and Hematology" notes that when caused by cancer, survival following diagnosis of a numb chin is about one year. The most common types of cancers to result in chin numbness are metastatic breast cancer and lymphoma. Prostate and lung cancers can also result in chin numbness.
Trauma and Infection
The nerves in your chin can be damaged by injuries to the jaw, for example in a motor vehicle accident. Dental procedures, in particular tooth extractions, can damage the inferior alveolar nerve, which ends in the mental nerve, thus causing numbness and tingling in the lower jaw and chin. An abscess in the lower gums can also lead to neuropathy, either of the inferior alveolar nerve, or of the mental nerve alone.
Chin numbness can be a sign of multiple sclerosis. Multiple sclerosis is a disease in which the myelin sheath covering nerve cells in the central nervous system is attacked. Symptoms include psychiatric problems, paralysis, loss of vision and sensory losses.