This paper examines how socially responsible investors' (SRIs) vertical cross-ownership of customer–supplier pairs can promote suppliers' green transitions and stimulate attention to supply chain issues of other portfolio firms. We exploit mergers in which high-E-score mutual funds (E-funds) acquire non-E-fund targets, creating exogenous cross-ownership when customer firms are in the acquiring E-fund and their suppliers are in the target fund. Using a stacked difference-in-differences approach, we find that cross-owned suppliers improve their environmental performance significantly post-merger. These improvements are driven by shifts in managerial attention and resources: acquiring funds become 9.7% more likely to mention supply chain issues in prospectuses and 10.4% more likely to appoint managers or trustees with relevant expertise. Moreover, the cross-ownership acts as a catalyst that increases E-funds' awareness of supply chain risks across their portfolios, generating spillover effects on other suppliers, including those of newly acquired and pre-existing customer firms. Overall, our findings highlight that through cross-ownership, SRIs can influence supply chain sustainability and generate meaningful spillover effects beyond directly held firms.