Building public support for climate mitigation policies is essential in making significant progress in decarbonization and achieving the global goal of net-zero by 2050. Understanding the role of social capital and network composition in fostering climate stewardship behaviors can provide valuable insights for developing such support. We conducted an online survey (n = 12,147) to analyze how social capital and homophilous/heterophilous social network diversity (SND) are associated with climate stewardship behaviors aligned with government recommendations. Social capital was significantly and positively associated with overall climate stewardship behaviors. Furthermore, homophilous SND exhibited stronger positive associations with overall climate stewardship behaviors compared to heterophilous SND. Nevertheless, the associations of homophilous and heterophilous SND with specific domains of climate stewardship behaviors were mixed. These findings underscore the critical importance of fostering and strategically leveraging social capital among individuals who share numerous, though not all, characteristics to promote climate stewardship behaviors in alignment with government climate mitigation guidelines.