Background: Uninsured adolescents and young adults (AYAs) and those with publicly funded health insurance are more likely to be diagnosed with cancer at later stages. However, prior population-based studies have not distinguished between AYAs who were continuously uninsured from those who gained Medicaid coverage at the time of cancer diagnosis. Methods: AYA patients (ages 15-39 years) with nine common cancers diagnosed from 2005 to 2014 were identified using California Cancer Registry data. This cohort was linked to California Medicaid enrollment files to determine continuous enrollment, discontinuous enrollment, or enrollment at diagnosis, with other types of insurance determined from registry data. Multivariable logistic regression was used to evaluate factors associated with later stages at diagnosis. Results: The majority of 52 774 AYA cancer patients had private or military insurance (67.6%), followed by continuous Medicaid (12.4%), Medicaid at diagnosis (8.5%), discontinuous Medicaid (3.9%), other public insurance (1.6%), no insurance (2.9%), or unknown insurance (3.1%). Of the 13 069 with Medicaid insurance, 50.1% were continuously enrolled. Compared to those who were privately insured, AYAs who enrolled in Medicaid at diagnosis were 2.2-2.5 times more likely to be diagnosed with later stage disease, whereas AYAs discontinuously enrolled were 1.7-1.9 times and AYAs continuously enrolled were 1.4-1.5 times more likely to be diagnosed with later stage disease. Males, those residing in lower socioeconomic neighborhoods, and AYAs of Hispanic or black race and ethnicity (vs non-Hispanic white) were more likely to be diagnosed at a later stage, independent of insurance. Conclusions: Our findings suggest that access to continuous medical insurance is important for decreasing the likelihood of late stage cancer diagnosis.