Plastic-film mulch is an agricultural technology widely used for maize (Zea mays L.) production in northern China, but the effects of the interaction between cultivar and plastic-film mulch on the grain yield and biomass production have not been studied. Five maize hybrids, Jinsui 4; Jixiang 1; Pingyu 8; Xianyu 335 and Yuyuan 5, were assessed in 2014 and 2015 in plastic-film mulched and non-mulched ridge-furrow plots in a cold semiarid environment. In 2014, the grain yield of the five cultivars varied from 685 to 898 g m−2 with no mulch, and from 966 to 1228 g m−2 under mulch. In 2015 (drier and warmer than in 2014), the grain yield varied from 179 to 353 g m−2 without mulch, and from 548 to 1021 g m−2 with mulch. By incorporating the variation in grain yield over cultivars, mulch treatments and cropping years, Jixiang 1 was the highest yielding cultivar: in 2014 the grain yield reached 898 g m−2 under no mulch, compared to 1228 g m−2 under mulch, while in 2015 the grain yield was 329 g m−2 under no mulch compared to 1021 g m−2 under mulch. Cultivar and mulch independently affected the aboveground biomass and harvest index of maize. Jixiang 1 attained high yields as a result of average aboveground biomass, but high harvest index. Differences in maize water use efficiency (grain yield per unit of evapotranspiration) as affected by mulch and cultivar paralleled those of grain yield due to minor effects of mulch and cultivar on growing-season evapotranspiration. The effects of plastic-film mulch on maize root biomass in the upper 0.6 m soil profile and root/shoot ratio varied with cultivar. Averaging years and mulch treatments, Jixiang 1 had the smallest root biomass in the upper 0.6 m soil layer and Jixiang 1 and Jinsui 4 had smaller root/shoot ratios than the other three cultivars. We conclude that the effects of plastic-film mulch on the grain yield, root biomass and root/shoot ratio varied significantly with cultivar. A moderate aboveground biomass and high harvest index may give satisfactory grain yield in plastic-film mulched or non-mulched fields in hydrothermally-limited areas. A wide variation of root/shoot ratio with maize cultivar and cropping year indicated that using a fixed root/shoot ratio to estimate the root carbon input to the soil would induce large uncertainty.