Background: Prior studies using cancer population data have shown that patients with Medicaid or no insurance (M/NI) have worse survival outcomes than other patients, likely due to differences in access to care, staging, and treatments received. However, little research has examined the relationship between patient insurance status and outcomes for patients enrolled to cancer clinical trials, who have uniform access to protocol guided treatment and are uniformly staged and treated. Methods: We examined survival outcomes for patients enrolled to phase III or large phase II clinical treatment trials conducted by the SWOG Cancer Research Network between 1992-2019. We compared patients with M/NI to all other patients with known insurance. Patients with military/VA insurance were excluded. Multivariable Cox regression frailty models were used, with cancer cohort as a random effect and covariate adjustment for age, race, and sex. We also adjusted for clinical risk by deriving a composite prognostic score based on key cancer-specific clinical risk factors. Separate analyses were conducted for those < 65 years vs. those 65 or older. Overall, progression-free, and cancer-specific survival outcomes were analyzed. Results: In total, 29,423 patients from 51 trials comprising 27 cancer-specific cohorts were examined. Overall, 31.1% of patients were ≥ 65 years, 64.0% were female, 9.6% were Black, and 11.6% had M/NI. For all three survival outcomes, patients with M/NI had statistically significantly worse outcomes in patients under the age of 65, including a 24% increased risk of death (HR = 1.24, 95% CI, 1.16-1.32, p <.001), a 12% increased risk of progression (HR = 1.12, 95% CI, 1.06-1.18, p <.001) and a 19% increased risk of cancer-specific death (HR = 1.19, 95% CI, 1.11-1.27, p <.001). Consistent findings were observed when comparing patients with Medicaid to all other patients, and separately, when comparing patients with no insurance to all other patients. In contrast, in those 65 or older, there was no statistically significant difference in overall, progression-free, or cancer-specific survival between patients whose insurance included Medicaid (i.e., dual eligible) vs. those with Medicare alone or Medicare plus private insurance. Conclusions: Access to protocol-guided therapy ameliorates much but not all of the insurance-related disparities in outcomes previously observed using cancer population databases. For those < 65 years, despite receipt of treatment in a clinical trial, remaining disparities for those with M/NI may be due to limited access to supportive services, worse access to post-protocol therapy, or management of non-cancer related conditions. In contrast, these factors are less likely to be relevant for individuals 65 or older receiving Medicare. These findings suggest that an individual’s health insurance can meaningfully impact cancer outcomes over the longer term.