There are very few longitudinal studies examining family functioning in early childhood. This was a prospective cohort study to identify maternal sociodemographic factors associated with family functioning over the first 5 years postpartum, using data from the Healthy Beginnings Trial. Family functioning was measured using the General Functioning Subscale of the McMaster Family Assessment Device. A total of 667 first-time mothers participated, of which 369 remained at 5 years. Family functioning did not change significantly over the 5 years. Mothers being Australian born, being employed, and having a partner were predictors for healthy family functioning. Targeting support for immigrants, low-income and single-mother families may therefore improve family functioning.