Aims: This study evaluated the influence of cultivar number in wheat cultivar mixtures on aboveground biomass and grain yield across two years with different rainfall. Methods: Field experiments were conducted in the 2014–2015 (wetter-than-average) and 2015–2016 (drier-than-average) growing seasons in northern China. The study contained either one cultivar (16 × monoculture) or combinations of cultivar mixtures containing two (8 × mixture-2), four (8 × mixture-4), eight (8 × mixture-8) or 16 (1 × mixture-16) wheat cultivars. Results: In the wetter year (2014–2015), grain yield and aboveground biomass in the cultivar mixtures did not differ from the monoculture mean and had no relationship with the number of cultivars across monocultures or cultivar mixtures. However, in the drier year (2015–2016), wheat grain yield and aboveground biomass had a significantly positive relationship with the number of cultivars across monocultures and cultivar mixtures, and both were significantly higher in the cultivar mixtures than the monoculture mean. The higher yields in cultivar mixtures were attributed to higher thousand-grain weights and spike numbers per unit area. The cultivar mixtures had more root biomass and root distribution at depth than the monoculture mean, which contributed to higher water use efficiencies in the drier year. Conclusions: Cultivar mixtures can improve crop performance under water-deficit conditions and are likely to buffer the negative effects of drought on wheat yield under global climate change.