Primary acquired melanosis (PAM) of the conjunctiva is a potentially serious melanocytic lesion that can lead to the development of a melanoma. A 60-year-old woman noticed pigmentation of the conjunctiva of her left eye for more than 10 years. She underwent excisional biopsy combined with cryotherapy and was diagnosed with PAM without atypia by intraoperative consultation. She was followed for 7 years, and no changes were observed. Fourteen years after the initial biopsy, she noted a growing conjunctival tumor, and a melanoma was suspected. She underwent orbital exenteration and skin grafting procedures. Histopathological examination of the specimen led to a diagnosis of conjunctival malignant melanoma. Re-examination of the initial biopsy specimen revealed that there was a proliferation of melanocytes that partially expanded over the basal layer of the conjunctiva which had been diagnosed as PAM with moderate atypia. We conclude that this case of conjunctival PAM had progressed to a conjunctival malignant melanoma after 14 years. Pathological evaluation of intraepithelial lesions has its limitations; thus, cases of PAM, even in the absence of obvious atypia, require careful follow-up.