Clinical trial efficacy and real-world effectiveness of oncological treatments can differ. This study assessed the real-world survival outcomes of first-line pembrolizumab plus chemotherapy per PD-L1 stratum in patients with metastatic non–small cell lung cancer (mNSCLC) and compared them to clinical trial results. All patients with nonsquamous and squamous mNSCLC who received first-line pembrolizumab plus chemotherapy in 7 Dutch teaching hospitals between January 1, 2019 and December 31, 2021 were included. Hazard ratios (HR) with confidence intervals (95% CI) for overall survival (OS) and progression-free survival (PFS) were estimated to determine the efficacy-effectiveness gap (EE gap) between real-world and clinical trial, stratified by PD-L1 stratum. The nonsquamous cohort (n = 486) consisted of 269 patients with PD-L1 < 1%, 158 with PD-L1 1% to 49%, and 59 with PD-L1 ≥ 50%. The squamous cohort (n = 117) consisted of 70 patients with PD-L1 < 1% and 47 with PD-L1 ≥ 1%. For OS, an EE gap was observed in nonsquamous patients with PD-L1 < 1% (HR 1.38 (95% CI 1.06-1.78; median OS 10 vs. 17.2 months) and HRs consistently >1 in all other nonsquamous and squamous PD-L1 strata, although not statistically significant. No EE-gap for PFS was observed in any stratum. No significant EE gap was found for pembrolizumab plus chemotherapy, except in the stratum nonsquamous mNSCLC with <1% PD-L1 tumor expression. In these patients, the survival in real-world was considerably shorter compared to the clinical trial results. Further studies are needed to determine which patient, treatment and or context factors contribute to this disparity. This study assessed the real-world survival outcomes of first-line pembrolizumab plus chemotherapy per PD-L1 stratum in nonsquamous (n = 269) and squamous (n = 117) mNSCLC patients and compared these outcomes to those observed in the clinical trial. Nonsquamous patients with PD-L1 <1% treated with pembrolizumab plus chemotherapy had considerably shorter OS in the real-world compared to the trial (10 vs. 17.2 months). No survival differences were observed in other PD-L1 strata.