Child mental health challenges increased during the COVID-19 pandemic and may be sustained by the current ‘stress context’ (e.g., unaffordability). Parent and child mental health are linked. Factors such as parenting practices may mediate this link, but results are mixed and research has only considered specific disorders. Transdiagnostic research examines broad symptom dimensions and physiological markers that may underly mental health challenges and may better capture how parenting practices mediate the link between parent and child mental health challenges. This thesis took a broad approach to evaluate how a variety of maternal mental health symptoms (i.e., depression, anxiety, sleep disturbances, anger) and physiological markers (i.e., heart rate, sleep duration, physical activity) were linked with child mental health in a sample of treatment-seeking mothers to young children (n = 137). How this association was mediated by harsh and positive parenting practices was also examined. Results supported the link between maternal mental health symptoms and child mental health problems, but did not find the same association for maternal physiology. While positive parenting was linked to child mental health, only harsh parenting mediated the association between maternal and child mental health with a medium-large effect size. This study adds to the literature as the first to take a broad approach to this problem and extends mediational findings to the first three years of life, a sensitive period (Wachs et al., 2014). Future research should extend this study across multiple timepoints to establish causality while also considering risk factors beyond maternal mental health.