Abstract Background Partnering and parenting are important life-stage transitions that often occur during young adulthood. Little is known about how these transitions affect two dietary behaviors linked to increased cardiometabolic disease risk: skipping breakfast and takeaway-food consumption. Objective Our aim was to examine whether partnering and parenting transitions during a 5-year period were associated with change in diet quality, skipping breakfast, and takeaway-food consumption. Design We conducted a cohort study. Questionnaires were completed at baseline (2004 to 2006) and follow-up (2009 to 2011). Marital status and number of children were self-reported. Participants/setting Australian participants (n=1,402 39% men) aged 26 to 36 years were included. Main outcomes measures Diet quality was assessed using a Dietary Guideline Index. Breakfast skipping (not eating before 9 am the previous day) and frequent takeaway-food consumption (≥2 times/week) were reported. Statistical analysis Linear regression (mean differences in Dietary Guideline Index) and log binomial regression (relative risks for skipping breakfast and frequent takeaway-food consumption) were adjusted for age, education, follow-up duration, day of the week (skipping breakfast only), the other transition, and baseline behavior. Results During the 5-year follow-up, 101 men and 93 women became married/living as married, and 149 men and 155 women had their first child. Diet quality improved among all groups and was similar at follow-up between those who experienced the transitions and those who did not. Compared to having no children, having a first child was associated with a lower risk of skipping breakfast for men (relative risk 0.65; 95% CI 0.42 to 1.01) and women (relative risk 0.47; 95% CI 0.31 to 0.72). Men who became partnered also had a lower risk of skipping breakfast than those who remained single (relative risk 0.64; 95% CI 0.42 to 0.98). The transitions were not significantly associated with takeaway-food consumption. Conclusions Life-stage transitions were not associated with better diet quality. Participants who became partnered or parents were more likely to eat breakfast at follow-up than those who remained single or had no children.