Low-input grassland biomass from marginal and other slightly more fertile sites can be used for energy production without competing with food or fodder production. The effect of grassland diversity on methane yield has received some attention, but we do not know how community assembly may affect methane yield from grassland biomass. However, methane yields determine the potential economic value of a bioenergy substrate. Hence, a better understanding of how plant community assembly affects methane yield would be important. We measured biomass production and methane yield in the second year of a grassland field experiment which manipulated the order of arrival of different plant functional groups (forbs, grasses or legumes sown first and all sown simultaneously) and sown diversity (9 vs. 21 species). The order of arrival of the plant functional groups significantly determined the relative dominance of each group which in turn mainly explained the variance in aboveground biomass production. Differences in area-specific methane yields were driven by differences in biomass production and which plant functional groups dominated a plot. When grasses were sown first, legumes and grasses codominated a plot and the highest area-specific methane yield was obtained. Overall, the results indicate that altering the order of arrival affected the community functional and species composition (and hence methane yields) much more than sown diversity. Our study shows that a combined use of positive biodiversity effects and guided plant community assembly may be able to optimize methane yields under field conditions. This may allow a guided, sustainable, and lucrative use of grassland biomass for biogas production in the future.