Poor childhood family conditions have a long-term effect on adult mental health, but the mechanisms behind this association are unclear. Our aim was to study the pathways from problematic family relationships in adolescence to midlife psychological distress via disadvantages in early adulthood. Participants of a Finnish cohort study at the age of 16 years old in 1983 were followed up at ages 22, 32 and 42 years old (N = 1334). Problems in family relationships were measured with poor relationship with mother and father, lack of parental support in adolescent's individuation process and poor home atmosphere, and mental health was assessed using Kessler's Psychological Distress Scale (K10). We analyzed the indirect effects of adolescent family relations on mental health at age 42 years old via various disadvantages (somatic and psychological symptoms, relationship/marital status, low education/unemployment and heavy drinking) at ages 22 and 32 years old. Problematic adolescent family relationships were associated with midlife psychological distress in women (0.19; 95% CI 0.11, 0.26) and men (0.13; 95% CI 0.04, 0.21). However, after adjustment for adolescent psychological symptoms, the association was only significant for women (0.12; 95% CI 0.04, 0.20). Poor family relationships were associated with various disadvantages in early adulthood. The association from poor family relationships (16 years old) to psychological distress (42 years old) was in part mediated via psychological symptoms in women (0.03; 95% CI 0.01, 0.04) and men (0.02; 95% CI 0.00, 0.04) and in women also via heavy drinking in early adulthood (0.02; 95% CI 0.00, 0.03). Adolescent family relationships have a role in determining adult mental health. Targeted support addressing psychological well-being and hazardous drinking for adolescents with problematic family relationships might prevent disadvantages in early adulthood, and further prevent poor midlife mental health.