The efficacy of osimertinib was compromised by the development of resistance mechanisms, such as EGFR C797S. In vitro study proved that cells harboring EGFR C797S in trans with T790M are sensitive to a combination of first- and third-generation EGFR tyrosine kinase inhibitors. However, this has not been reported clinically. We performed capture-based sequencing on longitudinal plasma samples obtained at various treatment milestones from a patient with advanced lung adenocarcinoma who was undergoing targeted therapy. At the development of resistance to osimertinib, the patient’s plasma sample revealed EGFR C797S located in trans with T790M. He achieved partial response accompanied by undetectable C797S after commencement of a combinatorial treatment consisting of erlotinib and osimertinib. After 3 months of progression-free survival, he experienced progressive disease with emergence of EGFR C797S located in cis to T790M. We report the first clinical evidence of efficacy generated by combination therapy consisting of first- and third-generation EGFR tyrosine kinase inhibitors targeting concomitant EGFR T790M and C797S in trans. We also reveal that the clonal progression of C797S from in trans to in cis at disease progression may serve as a potential resistance mechanism.