Background/Objectives: Small cell lung cancer (SCLC) is a highly metastatic and lethal malignancy. Doublet chemotherapy consisting of a platinum compound and etoposide had remained the standard first-line regimen for decades, particularly for the majority of patients, who are diagnosed with extensive-stage disease. Over the past 5 years, results from the IMpower133 trial led to the approval of anti-PD-L1 agent atezolizumab as part of the first-line regimen for extensive-stage SCLC. The accumulation of real-world evidence on atezolizumab efficacy in this context is essential for corroborating trial results and further improving patient outcomes. Herein, we present real-world evidence from a Bulgarian cohort receiving atezolizumab as part of first-line extensive-stage SCLC treatment, comparing these data to the results of IMpower133. Methods: Data were extracted from the electronic health records of patients receiving atezolizumab plus chemotherapy as first-line treatment for extensive-stage SCLC in the period between 1 January 2022 and 31 December 2024. Analysis was conducted via the Danny Platform, an analytic platform for the integration of real-world evidence using embedded machine learning and LLM algorithms. Patients matching IMpower133 inclusion criteria were included in a group for comparison with trial results. Results: A total of 302 patients were included in the analysis. The real-world progression-free survival (PFS) in our study surpassed that in IMpower133 (7.6 versus 5.2 months), in addition to higher PFS rates beyond six months. A lower Objective Response Rate (ORR) (38.7% versus 65.4%) was noted in our real-world cohort, which we attribute to real-world documentation practices. Conclusions: Real-world evidence regarding PFS and treatment response generally align with the efficacy of adding atezolizumab to first-line extensive-stage SCLC treatment reported in the IMpower133 trial, while reflecting differences that may arise in routine clinical practice.