Background <p>Limited research has explored the association between the lymphocyte-to-monocyte ratio (LMR) and mortality in patients with diabetes mellitus. We investigated the association of the LMR with both all-cause and cardiovascular mortality in individuals with diabetes mellitus.</p> Methods <p>This study enrolled participants from the 2003–2018 National Health and Nutrition Examination Survey (NHANES) cycles. Mortality data were extracted from the National Death Index records. Maximally Selected Rank Statistics (MSRS) was used to identify the best LMR cutoff that was significantly associated with the survival outcomes. Multivariate Cox regression and subgroup analyses were performed to investigate the correlations of the LMR with all-cause and cardiovascular mortality. Restricted cubic splines (RCS) analysis was used to depict the non-linear relationships of the LMR with all-cause and cardiovascular mortality. Time-dependent receiver operating characteristic (ROC) curve analysis was performed to assess the accuracy of the LMR for forecasting the survival outcomes.</p> Results <p>Over the median follow-up period of 76 months, 585 of 2,327 participants died, 180 of whom died of cardiovascular mortality. The participants were divided into two groups according to the MSRS: the low LMR (≤ 2.62) and the high LMR (&gt; 2.62) groups. The multivariate Cox regression analysis showed that the high LMR group had a significantly lower risk of all-cause mortality (hazard ratio [HR] 0.62, 95% confidence interval [CI] 0.50–0.76, <i>P</i> &lt; 0.001) and cardiovascular mortality (HR 0.55, 95% CI 0.38–0.81, <i>P =</i> 0.003) than the low LMR group. This trend remained consistent throughout the subgroup analyses, with no significant interaction (<i>P</i>_interaction &gt;0.05) observed between the LMR and these subgroup factors. The RCS regression analysis demonstrated positive non-linear relationships between the LMR and all-cause and cardiovascular mortality (both <i>P</i>_non−linear &lt; 0.05) in patients with diabetes mellitus. The area under the ROC curve (AUC) for all-cause mortality was 0.858, 0.807, 0.807, and 0.802 for 1-, 3-, 5-, and 10-year survival, respectively, and the AUC for cardiovascular mortality was 0.864, 0.800, 0.815, and 0.811, respectively.</p> Conclusion <p>In individuals with diabetes mellitus, high LMR correlated with a reduced risk of all-cause and cardiovascular mortality.</p>