Background: When discussing treatment options and future care, it is important to understand the expectations of patients and family caregivers related to palliative chemotherapy and to identify patterns in patients’ quality of life. The study aims were to evaluate differences in treatment expectations and quality of life between patients with thoracic cancer (non-small-cell lung cancer, small-cell lung cancer and mesothelioma) who were < 70 and ≥ 70 years of age and receiving palliative chemotherapy and to assess family caregivers’ treatment expectations. Methods: A prospective longitudinal study included patients with thoracic cancer receiving outpatient palliative chemotherapy at a university hospital in Denmark and their family caregivers. Patients’ treatment expectations and quality of life were assessed three times during treatment with a survey of treatment expectations and the Functional Assessment of Cancer Therapy – General questionnaire. Family caregivers’ treatment expectations were assessed once. Results: A total of 48 patients and 36 family caregivers participated between 2018 and 2019. No statistically significant age-related differences in treatment expectations and quality of life were identified. 28% of patients aged < 70 years and 7% of those aged ≥70 years expected a cure. Among family caregivers, 36% expected a cure. Across both age groups, mean total quality of life scores significantly decreased from 73.2 at first palliative chemotherapy cycle to 70.5 at third cycle (p = 0.02). No meaningful changes were found in quality of life within either age group. A subgroup analysis found no significant between-group differences in quality of life. Mean physical well-being score for all patients decreased from 20.3 at first cycle to 18.4 at third cycle (p = 0.03) and mean emotional well-being score decreased from 15.4 at first cycle to 14.6 at third cycle (p = 0.04). Conclusion: This study emphasizes the importance of initiating conversations about treatment expectations and paying attention to expectations that may differ by the age of the patient and between patients and family caregivers. Addressing treatment expectations among patients and family caregivers and monitoring quality of life among patients is important in clinical practice.