PURPOSE: We examined associations between caregiving intensity and mental health among cancer caregivers at the population level and potential moderation by an actionable intervention target, support service needs. METHODS: Behavioral Risk Factors Surveillance System survey data (2015) from caregivers of adult patients with cancer was analyzed. Caregiving intensity included hours per week caregiving (high, > 20; low, ≤ 20) and caregiving duration (long, > 2 years; short, ≤ 2 years). Mental health was reported as number of mentally unhealthy days (MUDs) in the past 30. Support service needs comprised caregiving classes, service access, support groups, counseling, and respite care. Multivariable linear regression models were performed adjusting for sociodemographics and sampling weights. RESULTS: A total of 1,831 caregivers were included in the study, representing approximately 1.1 million cancer caregivers in the 18 US states, distributed with the following intensity: 122 (8.3%) caregivers reported care at high hours/long duration, 213 (13.1%) high hours/short duration, 329 (18.4%) low hours/long duration, and 910 (60.2%) low hours/short duration. Mean MUDs was 6 (SE, 0.5). The highest reported unmet service need was help with service access (48.4%). Higher caregiving intensity and support service need were associated with more MUDs (P <.05), with a significant interaction (P =.02) between caregiving intensity and unmet support service needs. High hour/long duration caregivers reporting any unmet needs had a mean of 15 versus 8 MUDs for those with no unmet needs. CONCLUSION: High-intensity cancer caregiving was associated with poor mental health, especially for those reporting support service needs. Developing strategies to optimize support service provision for high-intensity cancer caregivers is warranted.