Background/Objectives: Despite advances in immunotherapy, reliable biomarkers beyond PD-L1 expression are urgently needed to optimize treatment decisions in advanced non-squamous NSCLC. Thyroid Transcription Factor-1 (TTF-1), a biomarker associated with favorable prognosis in chemotherapy-treated patients, has unclear prognostic implications in the immunotherapy era. Methods: We conducted a multicenter retrospective study involving 163 advanced non-squamous NSCLC patients treated with first-line immunotherapy or chemo-immunotherapy and an additional historical chemotherapy-only cohort (n = 37). We evaluated the prognostic significance of TTF-1 expression for progression-free survival (PFS) and overall survival (OS). A systematic review and meta-analysis, performed following PRISMA guidelines, integrated our findings with existing evidence. Hazard ratios (HRs) were calculated using Cox proportional hazards models. Results: TTF-1 negativity was associated with significantly worse median PFS (6.7 vs. 16 months; HR 2.22, 95% CI 1.59–3.13; p < 0.001) and OS (11.5 vs. 26.4 months; HR 2.33, 95% CI 1.64–3.45; p < 0.001) compared to TTF-1 positivity. The prognostic value of TTF-1 was independent of PD-L1 status, with limited predictive relevance of PD-L1 expression observed within TTF-1-negative tumors. Meta-analysis (9 studies, 14 cohorts, n = 2019 patients) confirmed significantly inferior outcomes for TTF-1-negative patients across multiple immunotherapy-based regimens (pooled HR for PFS: 1.75, 95% CI 1.50–2.04; OS: 1.76, 95% CI 1.45–2.14). Conclusions: TTF-1 negativity independently predicts poor prognosis in advanced non-squamous NSCLC treated with immunotherapy-based regimens, identifying patients with limited benefit despite high PD-L1 expression. Integrating TTF-1 status into clinical practice may guide personalized treatment strategies, highlighting the need for prospective validation.