Trans-10, cis-12 conjugated linoleic acid (CLA), a bioactive fatty acid, has the potential to alter energy partitioning in lactating cows by marginally reducing milk fat synthesis in the mammary gland. The objective of this study was to determine the effects of pre- and postpartum CLA supplementation on lactation performance in a commercial dairy setting. Holstein cows in a robotic milking system were blocked by multiparous (mp) or primiparous (pp), and expected calving date, and randomly assigned to either a CLA group (mp n = 100; pp n = 39) or control group (mp n = 98; pp n = 38). Cows were supplemented with 100g of lipid encapsulated CLA methyl esters (Lutrell Pure, BASF, Germany) mixed 50:50 with soybean meal to provide 10g each of trans-10,cis-12 CLA and cis-9,trans-11 CLA via a robot mineral supplement unit. Supplementation was from −21d precalving through 30d in milk (DIM) for mp or 70 DIM for pp. Milk yield, fat, and protein concentration were recorded daily and averaged by wk for the first 100 DIM. Data were analyzed using the MIXED procedure of SAS 9.4 with repeated measures. Treatment, wk, and treatment × wk were fixed effects with random effects of (cow)group. Means were considered different when P ≤ 0.1 and tended to differ when P ≤ 0.15. Daily milk yield over 100d was increased with CLA supplementation (100d mean mp: 49.7 vs. 51.3 ± 0.6 kg/d, P = 0.07; pp: 30.3 vs. 32.0 ± 0.7 kg/d P = 0.09) with the greatest increase in milk production being 3.0 kg/d for mp (wk 5; P = 0.007) and 3.9 kg/d for pp (wk 14; P = 0.001) cows. Supplementation with CLA did not alter milk fat concentration (P > 0.15) or yield (P > 0.15) over the 100 d period. Milk protein concentration was reduced (P = 0.05) with CLA supplementation in mp (3.18 vs. 3.12 ± 0.02%, control vs. CLA) but not changed (P > 0.15) in pp (3.27 vs. 3.22 ± 0.04%, control vs. CLA) cows; however milk protein yield was unaffected (P > 0.15). These results suggest that CLA supplementation during the transition to lactation period altered energy partitioning to increase milk production during the first 100 DIM.