Objectives I examined the relationship between paid maternity leave and maternal mental health among women returning to work within 12 weeks of childbirth, after 12 weeks, and those returning specifically to full-time work within 12 weeks of giving birth. Methods I used data from 3850 women who worked full-time before childbirth from the Early Childhood Longitudinal Study-Birth Cohort. I utilized propensity score matching techniques to address selection bias. Mental health was measured using the Center for Epidemiologic Studies Depression (CESD) scale, with high scores indicating greater depressive symptoms. Results Returning to work after giving birth provided psychological benefits to women who used to work full-time before childbirth. The average CESD score of women who returned to work was 0.15 standard deviation (p < 0.01) lower than the average CESD score of all women who worked full-time before giving birth. Shorter leave, on the other hand, was associated with adverse effects on mental health. The average CESD score of women who returned within 12 weeks of giving birth was 0.13 standard deviation higher (p < 0.05) than the average CESD score of all women who rejoined labor market within 9 months of giving birth. However, receipt of paid leave was associated with an improved mental health outcome. Among all women who returned to work within 12 weeks of childbirth, those women who received some paid leave had a 0.17 standard deviation (p < 0.05) lower CESD score than the average CESD score. The result was stronger for women who returned to full-time work within 12 weeks of giving birth, with a 0.32 standard deviation (p < 0.01) lower CESD score than the average CESD score. Conclusions The study revealed that the negative psychological effect of early return to work after giving birth was alleviated when women received paid leave.