Physical function (PF) in older patients with cancer may decline during and after oncologic therapy. This study aimed to develop and pilot test an individually tailored unsupervised physical activity (PA) program and dietary recommendations to promote PF in older patients with cancer. Following development and pretest, the intervention was pilot tested to explore feasibility, acceptance, adherence and potential benefit. Patients ≥60 years, with heterogeneous cancer diagnoses, starting outpatient radiotherapy were randomized in two study arms: paper-based vs. video-based instructions. Based on assessments of PF, PA, nutrition, cognition, mental health, social support, HRQOL and personal goals, participants received individual recommendations for PA and nutrition. After 12 weeks of intervention (T1), reassessments were performed. The postal 4-week follow-up questionnaire included PA, nutrition and HRQOL. Participants (n = 24, 14 female, mean age 70 ± 7 years) showed comparable characteristics in both study arms. The majority rated the program as helpful. Facilitators and barriers to PA adherence were collected. Both modes of instructions were appreciated equally. PF (EORTC QLQ-C30) declined slightly (not clinically relevant >10 pts.) at group level T0: 76 ± 16, T1: 68 ± 21, T2: 69 ± 24. The intervention was feasible, well accepted, showing potential benefit for the maintenance of PF during outpatient radiotherapy, and should be further tested in a larger sample.