Cultivation of legume–grass mixtures ensures a high, protein-rich forage yield with no nitrogen (N) fertilisation. The maintenance of a reasonable proportion of each species over time may depend on the variety. In mixtures of lucerne (Medicago sativa L.) and grass, the effect of genetic variation on biomass and N accumulation is little described. We analysed how lucerne genetic variation affects agronomic traits. The experiment included 46 lucerne and two tall fescue (Festuca arundinacea Schreb.) genotypes tested in microplots of three lucerne and four fescue clones, evaluating biomass production, plant height, stem number and N content in eight cuts in 2011 and 2012. There was a wide genetic variation among lucerne genotypes but no significant statistical interaction between lucerne and fescue genotypes. This suggests that agronomic value of lucerne genotypes for mixtures can be evaluated with any grass genotype. On average, the grass plants grown with highly productive lucerne genotypes had an increased leaf elongation and N status but a reduced tiller number, which could limit their persistence. This is the first observation that the choice of lucerne genotype determines morphology, biomass production and N absorption of both lucerne and tall fescue grown in mixture.