The objective of this study is to report the transformation of a 10-year history of melanotic mandibular gingival lesion into primary oral melanoma (POM) in a 37-year-old Taiwanese woman. The patient declined all treatments and died of widespread disease 15 months after POM was confirmed. The histology of initial biopsies was totally benign, but the size of lesion, irregular shape, and developing satellites of pigmentation at the periphery were clinically atypical. However atypical it was, this clinical appearance was unknown to the pathologists. This case demonstrates that oral melanoma can be preceded by a histologically benign-appearing flat pigmented lesion for an extended time. Therefore, a benign oral pigmentation should not be viewed as a harmless condition and regular periodical follow-up is necessary. We argue that the diagnosis and risk assessment of oral pigmented lesions require a team effort from both clinicians and pathologists to make an appropriate diagnosis and achieve a favorable prognosis.