Background: Geriatric patients (≥80 years) are underrepresented in immune checkpoint inhibitor (ICIs) clinical trials. However, their unique biology may affect their response to ICIs. There are currently no established biomarkers of the response to ICIs in adult patients with cancer that can help with patient selection. Methods: We built a multicenter, international retrospective study of 885 patients (p < 0.01 and SII-H, p < 0.05, respectively). At median follow-ups (13.03 months), and compared to SII-H, patients with SII-L had significantly longer median PFS and OS in patients p < 0.001), and ≥80 years (p < 0.001). SII-L was independently associated with longer PFS and OS (HR: 0.61 and 0.62, respectively, p < 0.01). Conclusion: Lower inflammation pre-ICI initiation may predict an improved response and survival in geriatric patients with cancer.