Background: EGFR ex20ins are distinct targetable driver mutations present in ~2% of all NSCLC. While most EGFR TKIs have limited effectiveness, mobocertinib is the first oral therapy approved for this unmet need. EGFR ex20ins are heterogeneous ( > 60 variants), and PCR testing detects a limited number of variants, leading to undetected disease in ~50% of cases, whereas NGS can identify all such mutations. Testing plasma ctDNA is a noninvasive approach for detection of genomic variants, such as EGFR ex20ins. Methods: In the mobocertinib phase 1/2 study (NCT02716116), patients were enrolled based on EGFR ex20ins detected by various local tissue or liquid clinical trial assays (CTAs). Baseline plasma samples were collected and processed to extract cell-free DNA and were evaluated for EGFR ex20ins status using FoundationOne Liquid CDX (F1LCDX), an NGS-based plasma ctDNA test. Concordance between F1LCDX and the CTAs was demonstrated by testing patient samples from the phase 1/2 trial and paired tissue and plasma samples from commercially acquired and stage-matched patients with NSCLC (N = 305). Efficacy analyses by assay method were performed in EGFR ex20ins–positive platinum-pretreated patients (PPP; n = 114 [FDA approved population]; data cutoff: 1 Nov 2021) in the phase 1/2 trial. Results: Concordance between F1LCDX and CTAs was demonstrated with samples from the CTA-positive (n = 159) and CTA-negative populations (n = 87) tested by F1LCDX where tissue and plasma were assessable; the point estimate of positive percentage agreement (95% CI) was 68.6% (61.0–75.3), and the point estimate of negative percentage agreement was 100% (95.8–100). F1LCDX detected 34 individual EGFR ex20ins variant types. In the response evaluable population, patients who tested EGFR ex20ins positive by F1LCDX (n = 55) demonstrated confirmed ORR (34.5%) and disease control rate (74.5%), which was comparable to those observed among all PPP in the phase 1/2 trial (confirmed ORR: 28%; confirmed disease control rate: 78%). Conclusions: Mobocertinib has shown efficacy in PPP with EGFR ex20ins NSCLC. F1LCDX effectively identified patients with EGFR ex20ins who may benefit from mobocertinib, providing an additional noninvasive diagnostic option to guide treatment. Clinical trial information: NCT02716116.