Background: Palliative care interventions improve quality-of-life for advanced cancer patients and their caregivers. The frequency and quality of service provision could be improved by a clinical tool that helps oncology professionals to assess unmet needs for palliative care interventions and to structure the interventions delivered. This paper aims to answer the following research question: what do oncology professionals and cancer patients view as important elements in a clinical tool for assessing unmet palliative care needs? Based on the feedback from professionals and patients, we developed and refined an intervention-focused clinical tool for use in cancer care. Methods: This study used a prospective convergent mixed methods design and was carried out at a single tertiary hospital in Switzerland. Healthcare professionals participated in focus groups (n=29) and a Delphi survey (n=73). Patients receiving palliative care were interviewed (n=17). Purposive sampling was used to achieve maximal variation in participant response. Inductive content analysis and descriptive statistics were used to analyze focus group discussions, open-ended survey questions and interview data. Descriptive statistics were used for analyzing quantitative survey items and interviewee characteristics. Results: Focus groups and Delphi surveys showed that seven key palliative care interventions were important to oncology professionals. They also valued a tool that could be used by doctors, nurses, or other professionals. Participants did not agree about the best timepoint for assessment. Two versions of a pilot clinical tool were tested in patient interviews. Interviews highlighted the divergent patient needs that must be accommodated in clinical practice. Patients provided confirmation that a clinical tool would be helpful to them. Conclusions: This paper reports on research carried out to understand what elements are most important in a tool that helps oncology professionals to identify patients' unmet needs and provide tailored palliative care interventions. This study demonstrated that professionals and patients alike are interested in a clinical tool. Responses from oncology healthcare professionals helped to identify relevant palliative care interventions, and patients provided constructive input used in designing a tool for use in clinical interactions.