Finding direction in new policy areas requires a combination of mandate, expertise, and stakeholder engagement. Here we investigate the formation of the EU's sustainable finance agenda through activity in and around its High Level Expert Group and Technical Expert Group. Actors from different professional ecologies struggle to determine the treatment of sustainable finance and establish policy practices. Those who shape issue treatments can be supported by a capacity to influence from either official mandate, scientific esteem, or claims to experience. These are contending conjectures to locate action among the professionals engaged in the process. We adjudicate between mandate, esteem, and experience with an assessment of the network ties and career histories of those involved in sustainable finance. Our findings suggest that those with many ties and mixed careers win. Professionals identified as offering access to a potential network of investors exhibit greater control over how issues are treated, to the detriment of civil society actors. We demonstrate likely influence over issue treatment through a discussion of environmental and social disclosures and debt financing mechanisms in European sustainable finance expert groups.