© 2019 Dionne-Odom et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Aims 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. Methods 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. Findings 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. Conclusions 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.