Rational: Epidermal growth factor receptor (EGFR) 20 exon insertion is the second most common EGFR aberrations in non-small cell lung cancer (NSCLC). Despite some novel EGFR inhibitors, clinically obtainable management for this subset of patients remains an unmet need. there are no previous reports of upfront combination therapy with immunotherapy and chemotherapy for lung adenocarcinoma with brain metastasis harboring EGFR 20 insertion. Patient concerns: A 56-year-old man who sought care for dry cough was diagnosed with lung adenocarcinoma with brain metastases indicating a poor prognosis. Diagnosis: Next-generation sequencing of lung biopsied tissue revealed an EGFR exon 20 in-frame insertion (P772_H773insYNP+H773Y). Interventions: The patient started treatment of pemetrexed and carboplatin plus programmed cell death-1 inhibitor sintilimab in November 2019. Outcomes: The patient achieved partial responses both intra- and extra-cranially. After 6 cycles of treatment, the patient accepted sintilimab plus pemetrexed every 3 weeks as maintenance therapy, which was well-tolerated without any toxicity and is still ongoing after 18 months since initiation of 1st-line treatment. Lessons: This is the first case report of the clinical benefit of upfront immune checkpoint inhibitors (ICIs) plus chemotherapy for a brain metastatic NSCLC patient harboring EGFR exon 20 insertion mutation. Further study is needed to validate the predictor involved in responders to ICIs-based therapy with EGFR mutations.