An 86-year-old woman was admitted to our hospital because of pain in the left iliac bone. Her whole-body computed tomography revealed left hilar lymphadenopathy, a mass on the right fifth rib, and osteolytic metastases on the left ilium. Microscopic findings in tumor samples from the left hilar lymph node obtained via endobronchial ultrasound-guided transbronchial needle aspiration revealed adenocarcinoma. She was accordingly diagnosed with lung adenocarcinoma (clinical stage IVB, T2bN1M1c) with multiple bone metastases. Molecular tests performed to detect driver mutations revealed a wild-type epidermal growth factor receptor and no anaplastic lymphoma kinase-1 or c-ros oncogene 1 translocations. The tumor proportion score for programmed death ligand 1 (PD-L1) of the tumor cells was 90%–100%. Although the patient had a performance status (PS) of 3, she wished to undergo active treatment. After receiving palliative radiotherapy for her left iliac bone metastasis, she received pembrolizumab as first-line treatment, having given fully informed consent. A partial response was observed after five cycles of pembrolizumab. Crucially, we were able to demonstrate the effectiveness of pembrolizumab as first-line treatment in a patient with non-small-cell lung cancer with a poor PS and high PD-L1 expression. To avoid missing an opportunity to treat such patients, the development of new biomarkers for detecting responders to cancer immunotherapy is required.