Background: Lung cancer remains the leading cause of cancer mortality with relatively few prognostic biomarkers. We investigated associations with overall survival for telomere length (TL) and genetic variation in chromosome 5p15.33, an established telomere maintenance locus. Methods: Leukocyte TL was measured after diagnosis in 807 patients with non–small cell lung cancer (NSCLC) from the Princess Margaret Cancer Center in Toronto and assessed prospectively in 767 NSCLC cases from the Copenhagen City Heart Study and the Copenhagen General Population Study. Associations with all-cause mortality were tested for 723 variants in 5p15.33, genotyped in 4,672 NSCLC cases. Results: Short telomeres (10th percentile) were associated with poor prognosis for adenocarcinoma in both populations: TL measured 6 months after diagnosis [HR ¼ 1.65; 95% confidence intervals (CI), 1.04–2.64] and for those diagnosed within 5 years after blood sampling (HR ¼ 2.42; 95% CI, 1.37–4.28). Short TL was associated with mortality in never smokers with NSCLC (HR ¼ 10.29; 95% CI, 1.86–56.86) and adenocarcinoma (HR ¼ 11.31; 95% CI, 1.96–65.24). Analyses in 5p15.33 identified statistically significant prognostic associations for rs56266421-G in LPCAT1 (HR ¼ 1.86; 95% CI, 1.38–2.52; P ¼ 4.5 10⁵) in stage I–IIIA NSCLC, and for the SLC6A3 gene with OS in females with NSCLC (P ¼ 1.6 10³). Conclusions: Our findings support the potential clinical utility of TL, particularly for adenocarcinoma patients, while associations in chromosome 5p15.33 warrant further exploration. Impact: This is the largest lung cancer study of leukocyte TL and OS, and the first to examine the impact of the timing of TL measurement. Our findings suggest that extremely short telomeres are indicative of poor prognosis in NSCLC.