Leptomeningeal carcinomatosis (LMC) is a known sequel of metastatic lung cancer and its treatment is challenging. Nevertheless, treatment options for LMC due to metastatic epidermal growth factor receptor-mutant (EGFR-mutant) lung adenocarcinoma are expanding. We present a 52-year-old male patient with metastatic non-small-cell lung cancer (NSCLC). The patient was found to have L858R mutation in exon 21 of the EGFR gene. He was initially treated with erlotinib, followed by afatinib/cetuximab, followed by chemotherapy. Thereafter, his disease progressed to LMC. Although tissue biopsy did not show T790M-mutation, osimertinib (160 mg once daily) promptly induced clinical and radiological response that continued for five months. High dose pulsed erlotinib (1500 mg weekly) improved his quality of life and extended his survival for a further four months.