There are few longitudinal studies on the mental health of parents with intellectual disabilities (ID). This study examined longitudinal trajectories of maternal stress for mothers with ID and borderline intellectual functioning (IF) compared to their peers without ID, over their children's ages. Additionally, it aimed to identify the impact of various individual, socioeconomic, and contextual factors on maternal stress. We used public data from the Future of Families and Child Wellbeing Study (FFCWS) between Years 1-15. Using mixed-effects multilevel modeling, we analyzed changes in, and predictors of maternal stress over time for mothers with ID (n = 89), borderline IF (n = 346), and without ID (n = 1,898). Maternal stress was especially high for mothers with ID and borderline IF when their children were ages 1-3. Across Years 1-15, mothers with ID and borderline IF reported higher levels of maternal stress compared to mothers without ID. Mothers who reported receipt of Social Security Income (SSI), greater levels of material hardship, and more children in the household reported higher maternal stress, whereas those who identified as Hispanic, were employed, married/partnered, and had greater social support reported lower maternal stress. Mothers with ID and borderline IF may experience elevated levels of chronic maternal stress when compared to their peers without ID. Effective early interventions, supportive programs, and policies that are tailored for mothers with ID and borderline IF who have younger children may be especially needed.