The aim of this study was to evaluate the efficacy of mepolizumab in patients affected by chronic rhinosinusitis with nasal polyps (CRSwNP) in real-life. A single-center retrospective observational study was conducted on severe CRSwNP patients treated with mepolizumab. Nasal endoscopic polyp score (NPS), visual analogue scale (VAS) symptom score, sinonasal outcome test (SNOT-22), asthma control test (ACT) score, fractional exhaled nitric oxide (FeNO), eosinophils blood cells and prednisone intake were assessed at baseline and after 6 months. A total of 55 patients were included; 49 patients (89%) presented with asthma; aspirin exacerbated respiratory disease (AERD) in 28 patients (51%). A statistically significant decrease in the SNOT-22 score was observed (median difference −63; 95% CI: −68; −58; p < 0.001) with median t0 76 and IQR (61;90) to t6 10 (5;15). A reduction in NPS, median t0 NPS 4; (IQR:4;6), median t6 NPS 1; (IQR:0;1) p < 0.001, was greater in patients with AERD. The median baseline VAS score was 6 (IQR:6;7) and the differences between t0 and t6 were statistically significant p < 0.001. Significant changes in blood eosinophils cells, median t0 500 cell/mcl (IQR:340;830), median t6 97 cell/mcl (IQR:60;160) p < 0.001, were greater in patients with AERD. Mepolizumab treatment effects have been demonstrated with significantly reduced symptoms, polyp scores, blood eosinophils and systemic corticosteroid use, resulting in an increased health-related quality of life in patients with severe CRSwNP, regardless of the presence or absence of asthma or AERD.