Western society has become increasingly obesogenic. Despite this trend, some individuals maintain low weights throughout their lives. Little is known about the characteristics of persistently thin individuals. This study examined demographic, health, and psychiatric characteristics of women who reported having been persistently thin throughout childhood, adolescence, and adulthood. Participants were 1022 female members of a population-based twin registry. Participants completed a mail survey that included questions regarding demographics, body size in childhood, adolescence, and adulthood, health behaviors and satisfaction, disordered eating, perfectionism, and personality. In previous interview waves, participants completed diagnostic interviews assessing psychiatric and substance use disorders. Persistent thinness was associated with a significantly later age at menarche, lower rates of dieting and binge eating, greater health satisfaction, higher self-esteem, and lower perfectionism and body dissatisfaction. Persistent thinness appears to be associated with greater well-being in the present and a later age of menarche in the past. Whether later menarche is causally related to thinness or whether later menarche, persistent thinness, and enhanced well-being all result from a third unmeasured variable such as high sports participation is yet to be resolved.