Background: During the vulnerable transition to parenthood, (expectant) parents may be particularly susceptible to the negative effects of adverse working conditions. However, research on the influence of work-related factors on peripartum mental health issues is scarce. This study aims to enhance our understanding of work-related risk factors for the adjustment of parents in the transition to parenthood by investigating the role of prepartum precarious employment, abusive supervision, job insecurity, and job demand on postpartum mental health symptoms in first-time mothers and their partners. Methods: In the prospective-longitudinal cohort study “DREAM”, N = 1,259 mothers and N = 811 male and female partners were asked about their working conditions during pregnancy and their mental health 14 months postpartum. We conducted several hierarchical multiple regression analyses with prepartum precarious employment, abusive supervision, job insecurity, and job demand (the latter three in joint regression analyses) as predictors of postpartum symptoms of depression, somatization, obsessive-compulsiveness, anxiety, and anger/hostility. In Model 1 we controlled for sociodemographic variables, and in Model 2 we also controlled for pre-existing symptoms of the respective mental health variable during pregnancy. Separate analyses were calculated for mothers and partners, and each mental health outcome. Results: Multiple regression analyses revealed that prepartum precarious employment and abusive supervision predicted mothers’ and partners’ mental health symptoms 14 months postpartum even after controlling for pre-existing symptoms. Prepartum job insecurity and job demand also predicted mental health symptoms 14 months postpartum but were no longer significant predictors in most models after controlling for pre-existing mental health symptoms. There were only minor differences regarding the considered mental health outcomes and between mothers’ and partners’ results. Conclusions: Our study demonstrates that adverse working conditions are important risk factors for the adjustment of parents in the transition to parenthood, requiring more attention from research and practice. Precarious employment and abusive supervision appear to be particularly important factors affecting new parents’ mental health. Future research should investigate the mechanisms behind these variables, including comparisons between mothers and their partners, and the role of stress-related biomarkers. Additionally, developing screening methods for clinical use to facilitate targeted preventive interventions is essential.