To improve their sustainability performance, firms are increasingly looking beyond their own internal operations towards managing their overall supply chain’s sustainability. In this study, we experimentally examine the joint effect of type of support buyers provide to their suppliers (tier-1 suppliers) and buyers’ sustainability product mix strategy on the extent to which tier-1 suppliers monitor the sustainability practices of their own suppliers (tier-2 supplies). We hypothesize and find that when buyers consistently integrate sustainable products into all of their product mix, tier-1 suppliers will monitor tier-2 suppliers to a greater extent when buyers subsidize tier-1 suppliers’ sustainability costs (direct support) than when buyers pay them a price premium (indirect support). In contrast, when buyers sell a mix of sustainable (at a premium) and conventional products, tier-1 suppliers monitor tier-2 suppliers more when buyers provide indirect support than direct support. Our findings show that buyers’ choice of supplier support and buyers’ product mix decisions can have important spillover effects on the sustainability performance further up the supply chain.