The postpartum period is critical for new mothers, in terms of performing maternal functions, which can be affected by physical or psychological complications. The purpose of the present study is to determine the relationship between maternal functioning and mental health in the postpartum period. This cross-sectional descriptive-analytic study was conducted on 530 eligible women who referred to health centers in Tabriz, Iran in 2018. The participants were selected through randomized cluster sampling, and data were collected by using a socio-demographic characteristics questionnaire, Mental Health Inventory (MHI), and the Barkin Index of Maternal Functioning (BIMF). These assessments were collected between 1 and 4 months postpartum. The relationship between maternal functioning and mental health was determined by conducting bivariate analysis via Pearson and Spearman correlation analysis and the general linear model (GLM) in a multivariate analysis. The mean (SD) mental health score in women was 79.1 (15.0) in the obtainable score range of 18 to 108, and the mean (SD) BIMF score in women was 97.4 (12.9) in the obtainable score range of 0 to 120. Based on Pearson or Spearman correlations, mental health and its sub-domains had positive, significant correlations with infant care, mother-child interaction, mental well-being, social support, management, adjustment, self-care, and maternal functioning (p < 0.001). Based on the GLM, increased maternal functioning was associated with higher total mental health score, having a moderate income, and receiving support for infant care (p < 0.05). High levels of postpartum mental health can have a positive impact on maternal functioning. Additionally, having support with infant care tasks can also improve functioning.