The objective of this study was to evaluate the effect of increasing dietary monensin (Rumensin, Elanco Animal Health, Greenfield, IN) supplementation on DMI, milk yield, and milk fatty acids (FA) in lactating dairy cows fed contemporary diets. One hundred ninety-two lactating dairy cows were balanced for milk yield, parity, and DIM and assigned to 12 pens housing 16 cows per pen with twelve multiparous and four primiparous cows. Diets were formulated for: 1) 0 g/ton monensin (g monensin per one ton of TMR; CON), 2) 11 g/ton monensin (R11), 3) 14.5 g/ton monensin (R14.5), and 4) 18 g/ton monensin (R18). All pens received the R11 diet for a 4-wk covariate period, then pens were balanced based on milk yield, milk components, and BW collected during the covariate period, and assigned to the treatment diets for the 9-wk experimental period. All diets were comprised of 35% DM corn silage, 19% grass haylage, 18% ground corn, 7.0% soybean meal, and 21% concentrate mix containing monensin. The statistical model included the fixed effects of treatment, week, and their interaction, and the random effect of pen nested within treatment. Contrasts were used to test the linear, quadratic, and cubic effects of increasing monensin supplementation. The analyzed monensin concentration of the treatment diets were 0 g/ton, 13 g/ton, 15.8 g/ton, and 19.3 g/ton monensin for R0, R11, R14.5, and R18, respectively. Increasing monensin supplementation tended to quadratically affect DMI, but did not affect the yields of milk, ECM, protein, fat, or lactose. Greater monensin supplementation linearly increased the concentration of de novo and mixed FA and tended to linearly increase the yields of de novo and mixed FA with no effect on preformed FA. Increasing monensin concentration quadratically affected ECM/DMI and FCM/DMI feed with a maximum observed at R14.5. These results suggest monensin can be supplemented in diets that support high milk component yields and milk production efficiency (marketable solids-corrected milk per unit of feed intake) in lactating dairy cows fed contemporary diets with the greatest response achieved at a dose of 14.5 g/ton monensin. The study also demonstrated a positive impact of monensin on de novo and mixed fatty acid synthesis, and thus further research is needed to understand the role of monensin in fatty acid synthesis.