Youth with mental illness struggle to receive essential behavioral health care. One obstacle is denial of coverage by insurance. In California, managed care consumers may apply for independent medical review (IMR) which potentially overturns an insurance denial through the California Department of Managed Healthcare (CDMHC). The authors aim to analyze IMR appeals for psychiatric treatment among adolescents and elucidate factors associated with obtaining coverage of care. We performed an analysis to identify factors that are associated with depression and substance use disorder (SUD) treatment claim denials in 11-20-year-olds from 2001 to 2022 using CDMHC data. Logistic regression modeling was used to identify specific factors related to claim characteristics and medical society instruments that are significantly associated with overturning a denial by IMR. Behavioral health IMRs are overturned at a higher rate than non-behavioral health claims. 54.5% of those with depression and 36.3% of those with SUD initially denied care coverage were overturned by IMR. For those seeking depression treatment, we found a significantly greater odds of overturn by IMR if there was a reference of CALOCUS [1.64, 95%CI (1.06-2.5)]. The odds of a SUD treatment denial being overturned was significantly greater if referencing CALOCUS [3.85 (1.54-9.62)] or ASAM [2.47, [4.3 (1.77-10.47)]. After the standardized implementation of illness severity tools in IMRs, the odds of a medically necessary claim being overturned was 2.5 times higher than before the standards. With a high percentage of claims being overturned after IMR, the findings suggest that health plans inappropriately deny medically necessary behavioral health treatment. The use of medical society instruments was associated with higher odds of overturning a denial. The recent decision of CDMHC to implement standard use of CALOCUS and similar illness severity criteria is supported by our findings and may facilitate more equitable care.