The link between childhood mental health difficulties such as conduct problems and adult political abstention has been overlooked despite early mental health difficulties potentially resulting in political self-marginalisation. Using the1970s British Cohort Study, we estimate developmental trajectories of conduct problems (i.e., from 5 to 16 years). Logistic regression, linear probability models, and propensity score matching were then conducted to test the association between trajectory group membership and voter turnout at 30, 42, and 46 years of age. Three distinct trajectories of conduct problems were identified: a normative (n = 11,871; reference group), moderate-chronic (n = 3433), and elevated-chronic (n = 250) group. Results revealed an association between conduct problems and decreased turnout. In particular the elevated-chronic group had a decreased odds of voting of 52.2%, 52.0%, and 45.7%, as compared to the normative group at 30, 42, and 46 years respectively. The moderate-chronic group had a decreased odds of voting of 24.7% as compared to the normative group at age 30 only. Matched results and linear probability models substantiated findings, suggesting (1) the importance of considering childhood factors when examining antecedents of lifelong voting behaviour, and (2) the political self-marginalisation of people with chronic childhood conduct problems more than 3 decades later.