Purpose <p>Do parents or childless adults have better mental health? The answer could vary by life phases, but past studies exploring the link between parenthood and mental health have not considered the entire life course. This study investigates the association between parenthood and mental health in an entire population and further looks at how parenthood interacts with factors such as age, gender, and socioeconomic status.</p> Methods <p>We used comprehensive Norwegian register data covering the entire population aged 31–80 years old (<i>N</i> = 2,234,087) to explore the relationship between parenthood and mental health conditions from 2006 to 2019. We used logistic regression models for population-wide analyses and conditional logistic models for analyses of discordant same-sex sibling sets and monozygotic twin pairs.</p> Results <p>Parenthood was associated with a lower risk of mental disorders (OR 0.67, 95% CI 0.67–0.68 for women; OR 0.60, 95% CI 0.59–0.60 for men), including depressive and anxiety disorders. Fathers also had a minor reduction in risk of symptoms related to mental illness (OR 0.98, 95% CI 0.97–0.98), while mothers had a slightly elevated risk of symptoms (OR 1.09, 95% CI 1.09–1.10). Overall, mental health disparities between parents and non-parents persisted throughout life and were more pronounced among men and those with low educational attainment. Our findings were consistent in sibling- and twin-matched analyses.</p> Conclusions <p>Our findings highlight parenthood as a significant indicator of mental health inequalities throughout adulthood and old age.</p>