early 15c., "written record," from Anglo-French memorie "note, memorandum, something written to be kept in mind" (early 15c., Old French memoire), from Latin memoria (from PIE root *(s)mer- (1) "to remember"). The more specific sense of "a notice or essay relating to something within the writer's own memory or knowledge" is from 17c. Meaning "person's written account of his or her life" is from 1670s. Related: Memoirist.
Biography, Memoir. When there is a difference between these words, it may be that memoir indicates a less complete or minute account of a person's life, or it may be that the person himself records his own recollections of the past, especially as connected with his own life; in the latter case memoir should be in the plural. [Century Dictionary]