Maternal postpartum mental health is influenced by a broad range of risk and protective factors including social circumstances. Forty percent of Australian women resume employment in the first year postpartum, yet poor quality employment (without security, control, flexibility or leave) has not been investigated as a potential social determinant of maternal psychological distress. This paper examines whether poor quality jobs are associated with an increased risk of maternal postpartum psychological distress. Data were collected from employed mothers of infants ≤12 months (n = 1,300) participating in the Longitudinal Study of Australian Children. Logistic regression analyses estimated the association between job quality and maternal psychological distress, adjusting for prior depression, social support, quality of partner relationship, adverse life events and sociodemographic characteristics. Only 21% of women reported access to all four optimal job conditions. After adjustment for known risk factors for poor maternal mood, mothers were significantly more likely to report psychological distress (adjusted OR = 1.39, 95% CI 1.09, 1.77) with each reduction in the number of optimal employment conditions. Interventions for maternal postpartum affective disorders are unlikely to be successful if major risk factors are not addressed. These results provide strong evidence that employment conditions are associated with maternal postpartum mood, and warrant consideration in psychosocial risk assessments and interventions.