Objective Although Loevinger's model of ego development is a theory of personality growth, there are few studies that have examined age‐related change in ego level over developmentally significant periods of adulthood. To address this gap in the literature, we examined mean‐level change and individual differences in change in ego level over 18 years of midlife. Method In this longitudinal study, participants were 79 predominantly White, college‐educated women who completed the Washington University Sentence Completion Test in early (age 43) and late (age 61) midlife as well as measures of the trait of Openness (ages 21, 43, 52, and 61) and accommodative processing (assessed from narratives of difficult life events at age 52). Results As hypothesized, the sample overall showed a mean‐level increase in ego level from age 43 to age 61. Additionally, a regression analysis showed that both the trait of Openness at age 21 and accommodative processing of difficult events that occurred during (as opposed to prior to) midlife were each predictive of increasing ego level from age 43 to age 61. Conclusions These findings counter prior claims that ego level remains stable during adulthood and contribute to our understanding of the underlying processes involved in personality growth in midlife.