Early lactation is an energy demanding period for dairy cows that threatens both animal health and milk yield. Metabolic changes occur in this period, affecting the liver, a key organ in gluconeogenesis and lipoprotein metabolism. We studied the impact of early lactation on hepatic mitochondrial function of dairy cows. Twenty-four Holstein-Friesian multiparous cows (664 ± 65 kg BW, 3.0 ± 0.4 BCS, spring calving) were assigned in a randomized block design to either (G0) total mixed ration (TMR) fed ad libitum (70% forage: 30% concentrate) or (G1) grazing plus supplementation from 0 to 180 d postpartum (DPP). The G1 cows grazed on Festuca arundinacea or Medicago sativa in one (6h) or 2 sessions (18h) depending on heat stress conditions 20–30 kgDM/cow^/day) and were offered TMR (15 kgDM/cow/day) or supplemented with 5.4 kgDM of a commercial concentrate, respectively. From 180 to 250 DPP, all cows grazed Festuca arundinacea (10h; 30 kgDM/cow/day) and were offered TMR (16 kgDM/cow/day). Liver biopsies were collected at 35 and 250 DPP and cryopreserved using sequential freezing in modified UW medium using 10% DMSO. Mitochondrial function was assessed measuring oxygen consumption rates using complex I and II substrates. Mitochondrial content was determined measuring citrate synthase (CS) activity. Data were analyzed as repeated measures in a mixed model including feeding strategy, DPP and their interaction. Maximum respiratory rate and oligomycin-sensitive respiration were unchanged in G0 (6.7 ± 1 vs. 7.0 ± 1, P = 0.6; 2.9 ± 0.7 vs. 2.5 ± 0.7 pmolO₂/min/mg, P = 0.5) during both early and late lactation and were lower in early lactation in G1 (2.8 ± 1 vs. 7.5 ± 1, P < 0.0001; 1.2 ± 0.7 vs. 2.7 ± 0.7 pmolO₂/min/mg, P = 0.03). However, no differences between groups were observed in CS activity (47 ± 4 vs. 43 ± 3, P = 0.5; and 43 ± 5 vs. 48 ± 3 mU/mg, P = 0.3; for G0 and G1 respectively suggesting that mitochondrial content did not vary. Our results indicate that mitochondrial function is impaired during early lactation in cows in the grazing system and not in the TMR system, which could be related to differences in plane of nutrition.