Women with type 2 diabetes mellitus are at a higher risk of pregnancy complications. Although traditional beliefs and practices influence diabetes management and breastfeeding, recommendations integrating Thai cultural beliefs in maternal care are lacking. The purpose of this study is to describe diabetes self-management in pregnancy and breastfeeding experiences in women with preexisting type 2 diabetes mellitus from Thailand. A convergent parallel mixed-methods study will be conducted. Data will be collected from 20 pregnant women with preexisting type 2 diabetes mellitus in Thailand who are either primigravida or multigravida, aged 20–44 years old, speak the Thai language, and provide consent. The National Institute on Minority Health and Health Disparities Framework’s sociocultural and behavioral domains guides the research aims. Data will be collected two times. The first time is during pregnancy (T1); study participants will complete questionnaires and engage in an interview about diabetes self-management, breastfeeding confidence, and breastfeeding intention. The second time is at 4–6 weeks postpartum (T2); study participants will be interviewed about their breastfeeding experiences. We will review and extract maternal health outcomes including body mass index, gestational weight gain, and glycated hemoglobin for T1 as well as fasting plasma glucose for T2. Qualitative data will be analyzed using directed content analysis. Quantitative data will be analyzed using descriptive statistics. Data sources will be triangulated with relative convergence in the results. This proposed study is significant because the findings will be used as a preliminary guide to developing a culturally tailored approach to enhance health outcomes of Thai women with diabetes in pregnancy and postpartum periods.