Purpose Older patient populations commonly have cognitive impairment, which might impact decisional capacity. We examined patients and family caregivers preferences for family involvement in treatment decision making assuming different level of cognitive impairment, and sought to explore the factors associated with the preferences and the degree to which patients and family members agree on preferences. Materials and Methods A total of 358 elderly cancer patient and caregiver dyads were recruited from the 11 cancer centers in Korea and were asked to express their preferences for family involvement in treatment decision making using hypothetical scenarios with three different levels of cognitive status (intact, mild impairment, and severe impairment). Results Both patients and family caregivers preferred greater family dominance in treatment decision making with the increasing the level of cognitive impairment (39.7%, 60.9%, and 86.6% for patients and 45.0%, 66.2%, and 89.7% for caregivers in each scenarios). Patient and family caregiver concordance in decisional control preference was small for all three scenarios (weighted k =0.32, k =0.26, and k =0.36, respectively). Higher patient education was associated with preference for patient dominance in treatment decision in conditions of both mild and severe cognitive impairment. The association of higher patient education and patient-caregiver preference concordance was positive with intact cognition, while it was negative with severe cognitive impairment. Conclusion Decision control preferences were affected by hypothesized cognitive status of the patients. Findings from our study would be helpful to develop effective strategy for optimizing family involvement in cancer treatment decision in the context of deteriorating cognitive function of the patients.