Objective: Family caregivers (FCs) of cancer patients often experience high distress. This randomized clinical trial assessed the feasibility and preliminary effects of an intervention to improve FC supportive care. Method: A pragmatic and minimal intervention to improve FC supportive care was developed and pretested with FCs, oncology team, and family physicians to assess its relevance and acceptability. Then, FCs of lung cancer patients were randomized to the intervention or the control group. The intervention included (1) systematic FC distress screening and problem assessment in the first months after their relative cancer diagnosis, and every 2 months after; (2) privileged contact with an oncology nurse to address FC problems, provide emotional support and skills to play their caregiving role; (3) liaison with the family physician of FCs reporting high distress (distress thermometer score ≥4/10) to involve them in the provision of supportive care. Distress, the primary outcome, was measured every 3 months, for 9 months. Secondary outcomes included quality of life, caregiving preparedness, and perceived burden. At the end of their participation, a purposive sample of FC from the experimental group was individually interviewed to assess the intervention usefulness. Content analysis was performed. Results: A total of 109 FCs participated in the trial. FC distress decreased over time, but this reduction was observed in both groups. Similar results were found for secondary outcomes. However, FCs who received the intervention felt better prepared in caregiving than controls (p = 0.05). All 10 interviewed FCs valued the intervention, even though they clearly underused it. Knowing they could contact the oncology nurse served as a security net. Significance of results: Although the intervention was not found effective, some of its aspects were positively perceived by FCs. As many of them experience high distress, an improved intervention should be developed to better support them.