INTRODUCTION: Functional outcomes during non-small cell lung cancer (NSCLC) treatment are critically important to older adults. Yet, data on physical function and which measures best capture functional change remain limited. MATERIALS AND METHODS: This multisite, mixed methods cohort study recruited adults ≥65 years with advanced NSCLC starting systemic treatment (i.e., chemotherapy, immunotherapy, and/or targeted therapy) with non-curative intent. Participants underwent serial geriatric assessments prior to starting treatment and at one, two, four, and six months, which included the Karnofsky Performance Scale (KPS, range: 0-100%), instrumental activities of daily living (IADL, range: 0-14), European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire Physical Functioning subscale (EORTC QLQ-C30 PF, range: 0-100), and Life-Space Assessment (LSA, range: 0-120). For all measures, higher scores represent better functioning. In a qualitative substudy, 20 patients completed semi-structured interviews prior to starting treatment and at two and six months to explore how treatment affected their daily functioning. We created joint displays for each interview participant that integrated their longitudinal KPS, IADL, EORTC QLQ-C30 PF, and LSA scores with patient quotes describing their function. RESULTS: Among 87 patients, median age was 73 years (range 65-96). Mean pretreatment KPS score was 79% (standard deviation [SD] 13), EORTC QLQ-C30 PF was 69 (SD 23), and LSA was 67 (SD 28); median IADL was 13 (interquartile range [IQR] 10-14). At two months after treatment initiation, 70% of patients experienced functional decline on at least one measure, with only 13% of these patients recovering at six months. At two and six months, decline in LSA was the most common (48% and 35%, respectively). Joint displays revealed heterogeneity in how well each quantitative measure of physical function captured the qualitative patient experience. DISCUSSION: Functional decline during NSCLC treatment is common among older adults. LSA is a useful measure to detect subtle functional decline that may be missed by other measures. Given heterogeneity in how well each quantitative measure captures changes in physical function, there is value to including more than one functional measure in geriatric oncology research studies.