The impact of clinical stage on the effectiveness of osimertinib for epidermal growth factor receptor (EGFR) mutation-positive non-small cell lung cancer (NSCLC) remains unexamined. We investigated osimertinib therapeutic efficacy variation between stage IVA or lower and stage IVB EGFR mutation-positive lung cancers, focusing on differences in pretreatment co-occurring genetic alterations in circulating tumor DNA. This was a secondary analysis of the ELUCIDATOR study, a multicenter prospective observational study in Japan that assessed the mechanisms underlying resistance to osimertinib as a first-line treatment for advanced NSCLC with EGFR mutations. We compared the progression-free survival (PFS), overall survival (OS), and pretreatment co-occurring genetic alterations detected in plasma between patients with stages IVA (n = 83) and IVB disease (n = 84). Multivariate analysis of PFS and OS revealed that stage IVB was associated with a poor prognosis (hazard ratio [HR]: 2.03, 95% confidence interval [CI]: 1.36-3.04, p < 0.001, HR: 2.35, 95% CI: 1.45-3.90, p < 0.001, respectively). Pre-osimertinib treatment, significantly more TP53- (52.4% vs. 27.7%, p = 0.002), EGFR amplification- (58.3% vs. 23.2%, p < 0.001), and MET amplification-positive cases (22.6% vs. 7.2%, p = 0.008) were found among stage IVB than among stage IVA or lower cases. Patients with EGFR-positive NSCLC in stage IVB exhibited significantly shorter PFS and OS than those in earlier stages when treated with first-line osimertinib. The prevalence of baseline TP53 mutations, EGFR amplification, and MET amplification in plasma were significantly higher in stage IVB cases, implicating them in the worse outcomes of this group.