A black mole on the neck may be caused by pigmented nevus, melanoma, seborrheic keratosis and so on. 1. Pigmented nevus: it is a benign skin tumor of melanocyte origin, which can be divided into congenital and acquired, and can occur in any part of the body’s skin, so it can lead to nevus on the neck, and the appearance of the damage is a flat or slightly elevated black-brown or reddish-brown patches or maculopapular rashes. 2. Melanoma: it is a tumor formed by malignant transformation of melanocytes, which can occur in skin, mucous membrane, uvea of eyes, soft meninges and other parts of the body, and may also lead to nevus on neck. 3. Seborrheic keratosis: it is the most common benign skin proliferative tumor in old age, the tumor cells are mainly derived from the pigment formed by keratin, which may be related to chronic inflammatory stimulation by sun exposure, characterized by one or several yellowish, light brown flat papules, or plaques, and it may also lead to long dark nevus on neck. Neck long mole, there may be other reasons, need to go to the regular hospital in time to check, clear cause after standardized diagnosis and treatment.