Numerous nations have implemented lockdown measures in response to the COVID-19 pandemic. As a consequence of the lockdown on daily living, social participation, and health service accessibility, vulnerable people, for example, new mothers, may experience an increase in mental health problems. This cross-sectional survey was conducted to investigate the impact of the COVID-19 pandemic lockdown on Thai new mothers and the variables affecting their mental health. The survey data were collected from 903 Thai mothers with infants aged 0–12 months using an online platform and a face-to-face interview questionnaire survey between 17 July and 17 October 2020, during the first nationwide COVID-19 lockdown period. For the final analysis, there were 862 participants who completed all of the questions. The full exploratory analysis was performed by multivariable linear regression to identify the variables influencing maternal mental health. Our study demonstrated that new mothers reported feeling a high extent to some extent of worry (44.9%), increased appetite (40.4%), becoming easily annoyed or irritable (39.1%), and feeling down (33.5%), whereas 82.7% felt able to cope with the first lockdown situation. Practiced relaxation techniques were associated with positive maternal mental health (adjusted β = 1.05, 95% CI 0.57 to 1.52, p < 0.001). The perceived impact of the COVID-19 lockdown was on the household’s ability to pay for rent, to make mortgage payments (adjusted β = −1.59, 95% CI −2.87 to −0.36, p = 0.011), the household’s ability to pay for other essentials, such as utilities and medication (adjusted β = −1.99, 95% CI −3.16 to −0.81, p = 0.001), household crowding after lockdown (adjusted β = −3.46, 95% CI −4.86 to −2.06, p < 0.001), and not going outside or doing outdoor activities (adjusted β = −2.22, 95% CI −3.35 to −1.08, p < 0.001). These impacts were significantly associated with negative mental health. In conclusion, our results emphasize the critical need for continuous monitoring of maternal mental health and developing an effective response strategy and activity for promoting maternal mental health under the stress of repetitive lockdowns and increased economic pressures.