An increasing awareness of sustainability problems urges the need for more sustainable practices within the infrastructure industry. Organisations actively engage sustainability in strategic missions and objectives. However, how these visions translate proactively to the tactical and operational levels of decision-making is still considered an exploratory area for the extant theoretical body of knowledge. Therefore, this research aims to support managers in tactical and operational decision-making for sustainability by means of a diagnostic research in the context of a long-term Strategic Partnership between client and contractor at Schiphol Airport. Their collaborative agreements based on Best Value aims for synergy by close cooperation between the partners and valuing professional expertise. To become highly committed to sustainability, organizations need to take responsibility over decisions regarding their organisational processes, in terms of dedication, motivation, and long-term investments to achieve effective solutions. Interventions require the direction of efforts into an organization-wide approach, meaning that both tactical and operational level management are internally aligned and stimulated to involve sustainability in their considerations. Also, a future directed mind-set and long-term investment approach are required to overcome technical and respectively cost related barriers. Fragmented decision-making can be overcome by engagement of stakeholders in sustainability ambitions. Moreover, proactivity in strategic partnering collaborations also requires extending responsibility towards supporting the partner’s responsibilities in order to achieve the best outcome for all parties involved. Therefore, it is key for both parties to allow each other to showcase their expertise within the set boundaries and ambitions of the Best Value principles. Currently, the client initiates new projects in which utility infrastructure projects play a role, whereas the contracting organization becomes involved after scope, budget, and planning are mainly determined. A mutually structured approach of formal meetings in the initiation process needs to be enhanced to collectively explore opportunities and define ambitions. Based on the gathered elements on proactive implementation of sustainability in decision-making levels, a conceptual framework is developed to test the case studies by analyzing sustainable outcomes in terms of their level of effectiveness. The analysis framework introduced by this study connects the different levels of proactivity with a chronological order of project phases, ranging on tactical and operational decision-making levels. This developed framework provides a practical solution for the collaborative setting between client and contractor, as it stimulates the development of mutual ambitions, substantive opportunities and directions for sustainable outcomes.