Background: The clinical utility of immune checkpoint inhibitor (ICI) has been well established for CTLA-4 and PD-1/PD-L1 axis. A new ICI targeting LAG3 expressing T-cells was recently approved in combination with PD-1 inhibitor for advanced melanoma, with studies underway in other cancers. In this study, we assess LAG3 expression in a pan-cancer cohort, co-expression with other known ICI targets, and the survival and response implications of LAG3 expression in patients with non-small cell lung cancer (NSCLC) treated with pembrolizumab. Methods: A discovery cohort (DC) of 15,630 FFPE tumors of 35 histologies was evaluated by comprehensive immune profiling (CIP). LAG3 expression, measured by RNA-seq, was labeled as high (rank ≥75), moderate (rank 25-74), and low (rank < 25), and compared with tumor inflammation and cellular proliferation (expression of cellular proliferation markers such as ki-67) phenotypes. Co-expression analysis with other ICI targets was performed using Spearman correlation (r_s) with p-values reported. In a retrospective cohort (RC) of 72 metastatic NSCLC patients treated with pembrolizumab alone, Kaplan-Meier analysis was performed to test for differences in overall survival (OS) and progression free survival (PFS). Differences in objective response rate (ORR) were determined using chi squared test. Results: A majority of patients in the DC had moderate LAG3 expression (46.63%). LAG3 high was most predominant in gynecological cancers such as uterine (42%), ovarian (39%) and cervical cancers (37%). High LAG3 was associated with high cellular proliferation (p < 0.0001), regardless of tumor inflammation. LAG3 was significantly co-expressed with other ICI targets such as PD-1 (r_s> 0.6, p < 0.05), PD-L1 (r_s> 0.4, p < 0.05), PD-L2 (r_s> 0.6, p < 0.05) and TIM3 (r_s> 0.5, p < 0.05). This finding was supported by significant association of PD-L1 by immunohistochemistry and LAG3 expression (p = 0.002) in the RC. 15% of DC was both PD-1 high and LAG3 high (2344/15,630). Similar results were observed for LAG3 high and PD-L1 high (1870/15,630; 12%). LAG3 high patients in RC showed significantly higher OS (median OS = not reached; p = 0.016) and PFS (median PFS = not reached; p < 0.0001) compared the LAG3 low patients (median OS = 12 months; median PFS = 6.5 months). LAG3 high cases had significantly (ORR = 65%, p = 0.005) improved ICI response compared to LAG3 low cases (ORR = 35%). Conclusions: Our study showed a wide dynamic range of LAG3 expression across a pan-cancer cohort of solid tumors, with highest prevalence observed in gynecological cancers. Additionally, LAG3 is strongly associated with proliferation and significantly improved outcomes for patients with NSCLC, treated with pembrolizumab alone. Furthermore, significant LAG3 co-expression with other ICI targets suggests its plausible use in clinical trial selection and patient stratification for combination immunotherapy strategies.