Unmarried mothers living in residential facilities (UMLFs) in Korea face complex and challenging physical, psychological, and socioeconomic issues. This study developed a physical and mental health promotion program using urban forests for UMLFs based on the transtheoretical model and evidence. We utilized an intervention mapping approach (IMA) and assessed the needs of UMLFs by analyzing previous quantitative studies. Moreover, we conducted a qualitative hermeneutic phenomenological study involving nine participants. Based on the needs assessment, important and changeable determinants were identified; further, the program performance and change objectives were classified to achieve the program goals and establish the intervention strategy. We found that physical activity using forests, self-reflection using metaphors, five-sense activities, achievement activities using natural objects, building interpersonal relationships in the forest, and designing future plans, are desirable methods for improving the health of UMLFs. The IMA was deemed appropriate for the systematic development of health promotion programs for UMLFs through clear links among change objectives, theoretical methods, and practice strategies. These results should be applied to future intervention studies.