The aim of this study was to evaluate the effect of two winter diets (WD) (100 F, i.e. 100% forage, 8.3 kg DM of lucerne hay + 0.3 kg DM of straw; and 65F:35C, i.e. 65% forage:35% concentrate, 5.4 kg DM lucerne hay + 0.3 kg DM straw + 3.0 kg DM barley), offered during 118 days on meat quality traits of 20-month old steers finished on mountain pasture supplemented with 4.1 kg DM barley d–1. Longissimus thoracis intramuscular fat content and its fatty acid profile were determined (in vivo) after one month on pasture. The intramuscular fat content, fatty acid profile, texture (1, 8 and 15 days of ageing), colour (1, 2, and 8 days of oxygen exposure) and sensorial quality (8 and 15 days ageing) were evaluated post-mortem after 163 days on the finishing diet. Intramuscular fat content and fatty acid profile were affected by the WD in vivo (p < 0.05) but not post-mortem. Meat pH was not affected by the WD but the texture was affected by the interaction between the WD and the ageing time (p < 0.001), maximum stress decreased more rapidly in the 100F diet in the first 8 days of ageing. Meat colour was only affected by the oxygen-exposure time (p < 0.001). Panel test variables were not affected by the WD, but ageing time affected beef flavour intensity (p < 0.05). In conclusion, the winter diet affected intramuscular fat content after one month of grazing but had no major effects on post-mortem meat quality of pasture-finished steers.