This mixed methods study sought to assess adolescent and young adult (AYA) adherence to three cancer treatment recommendations (medications, diet, physical activity), and determine the individual, family, and health system factors associated with suboptimal adherence. In Stage 1, 72 AYA-caregiver dyads completed a validated adherence interview and surveys about individual and family functioning. Matched providers ( = 34 who reported on 61 AYAs) completed global adherence ratings through survey. In Stage 2, a subset ( = 31) completed qualitative interviews. Medication adherence was higher ( = 94.8%) than diet ( = 73.9%) and physical activity ( = 55.4%), although ≥50% demonstrated “Imperfect Adherence” for each subtask. Univariately, AYAs who missed a medication had more depressive symptoms, worse health-related quality of life (HRQOL), and more medication barriers; their families had more financial stress, worse family functioning, and lower self-efficacy. The odds of adhering to medications were lower with worse HRQOL (odds ratio [OR] = 1.08; 95% confidence interval [CI], 1.02-1.15) and family functioning (OR = 0.18; 95% CI, 0.04-0.91). The odds of adhering to physical activity and diet were lower with worse family functioning (OR = 0.09; 95% CI, 0.01-0.91) and more barriers (OR = 0.24, CI: 0.10-0.61), respectively. Qualitative themes further supported multilevel influences on AYA adherence. Adherence challenges were identified across medications, diet, and physical activity. Multilevel contextual factors were associated with suboptimal adherence, including poorer HRQOL and family functioning. Findings support the need to improve clinical adherence assessment and develop contextually tailored interventions.