Background: Adults with cancer who are pre-frail or frail are at risk of poor outcomes. Geriatric assessment (GA) is recommended to assess and manage vulnerability and risk of frailty in older adults with cancer (≥65) and to inform referrals in supportive services, including rehabilitation. Yet, adoption of the GA in community oncology practice lags, and frailty among adults younger than 65 often goes undetected and/or unaddressed. We evaluated the feasibility of a GA-guided rehabilitation care model and assessed changes in patient-reported and performance-based outcomes after rehabilitation. Methods: Adults (≥18 years) starting systemic therapy at a community oncology practice enrolled in the study. The GA was administered online and monthly for one year. Frailty/pre-frailty was identified using a previously validated 44-item index. The oncology team was notified of frail/pre-frail patients and then made referrals to outpatient rehabilitation. Feasibility outcomes (recruitment, retention, fidelity) and participant acceptability [7 items, 0–5 Likert scale] were analyzed descriptively. Patient-reported and performance-based outcomes were examined using the paired t-test. Results: 48% of eligible patients enrolled (N = 141), and 83% completed at least one GA. Frailty/pre-frailty was identified in 40% of the GAs, resulting in 282 referrals to rehabilitation (99% fidelity). Acceptability scores ranged from 3.5 ± 1.7 to 4.7 ± 0.6. Participants who attended rehabilitation (52%) improved significantly in outcomes measuring health-related quality of life, mobility, aerobic capacity, and strength (all p < 0.05). Conclusion: Implementing a GA-guided rehabilitation care model was feasible and acceptable to patients receiving systemic treatment. Those who attended rehabilitation experienced significant improvement in patient-reported and performance-based outcomes.