Background <p>It is well-known that mothers caring for children with developmental disabilities (DD) commonly suffer from social isolation. Connecting with other parents may play a critical role in the lives of mothers caring for children with DD.</p> Methods <p>We conducted a cross-sectional survey of 1,358 mothers caring for children with DD who are under 19 in 2024. Caregiving duration was defined as years since diagnosis and categorized into five groups: 0–3, 4–6, 7–9, 10–12, and 13–18 years. Depressive symptoms and burnout were measured by CES-D 11 and Copenhagen Burnout Inventory, respectively. Parents’ group participation was defined as attending disability-related parents’ group activities at least once a month. The association between caregiving duration and mental health was examined, stratified by group participation.</p> Results <p>The association differed by whether mothers participated in parents' groups. Among participants, compared to the 0–3 years group, the prevalence ratios for depressive symptoms were 0.81 (95% CI: 0.68–0.97) for 4–6 years, 0.77 (95% CI: 0.61–0.97) for 7–9 years, 0.77 (95% CI: 0.60–1.00) for 10–12 years, and 0.74 (95% CI: 0.56–0.97) for 13–18 years. However, no association was observed among non-participants. Similar differential patterns were found in the analysis of burnout.</p> Conclusions <p> Participation in parents’ group may play a potential buffering role in the association between caregiving duration and mothers' mental health.</p>