Collaboration and team science are emerging areas of interest in research and software production. Historically, multi-institutional research collaborations are difficult to initiate and maintain, negatively impacting communication, negotiation, and dialogue between industry, government, and academic researchers. The Exascale Computing Project was created to facilitate broader research collaboration under a shared funding structure and extended timeline to support scientific discovery. We conducted interviews with ECP teams, representing a variety of domain specialities, research institutions, programming backgrounds, and application areas. Using grounded theory as our analytical framework, we assessed how ECP’s structure created an environment of increased trust between projects and how software shared between teams facilitated sustained collaboration. Based on our findings within ECP projects, we share recommendations for sustainable multi-institutional collaboration and best shared software practices.