Numerous healthcare decisions are faced by persons with advanced cancer from diagnosis to end-of-life. The family caregiver role in these decisions has focused on being a surrogate decision-maker, however, little is known about the caregiver's role in supporting upstream patient decision-making. We aimed to describe the roles of family caregivers in assisting community-dwelling advanced cancer patients with healthcare decision-making across settings and contexts. Qualitative study using one-on-one, semi-structured interviews with community-dwelling persons with metastatic cancer (n = 18) and their family caregivers (n = 20) recruited from outpatient oncology clinics of a large tertiary care academic medical center, between October 2016 and October 2017. Transcribed interviews were analyzed using a thematic analysis approach. Caregivers averaged 56 years and were mostly female (95%), white (85%), and the patient's partner/spouse (70%). Patients averaged 58 years and were mostly male (67%) in self-reported "fair" or "poor" health (50%) with genitourinary (33%), lung (17%), and hematologic (17%) cancers. Themes describing family member roles in supporting patients' upstream healthcare decision-making were: 1) seeking information about the cancer, its trajectory, and treatments options; 2) ensuring family and healthcare clinicians have a common understanding of the patient's treatment plan and condition; 3) facilitating discussions with patients about their values and the framing of their illness; 5) posing "what if" scenarios about current and potential future health states and treatments; 6) addressing collateral decisions (e.g., work arrangements) resulting from medical treatment choices; 6) originating healthcare-related decision points, including decisions about seeking emergency care; and 7) making healthcare decisions for patients who preferred to delegate healthcare decisions to their family caregivers. These findings highlight a previously unreported and understudied set of critical decision partnering roles that cancer family caregivers play in patient healthcare decision-making. Optimizing these roles may represent novel targets for early decision support interventions for family caregivers.