Due to high magnitude and variation in spending on post-acute care, accountable care organizations (ACOs) are focusing on transforming management of hospital discharge through relationships with preferred skilled nursing facilities (SNFs). Using a mixed-methods design, we examined survey data from 366 respondents to the National Survey of ACOs along with 16 semi-structured interviews with ACOs who performed well on cost and quality measures. Survey data revealed that over half of ACOs had no formal relationship with SNFs; however, the majority of ACO interviewees had formed preferred SNF networks. Common elements of networks included a comprehensive focus on care transitions beginning at hospital admission, embedded ACO staff across settings, solutions to support information sharing, and jointly established care protocols. Misaligned incentives, unclear regulations, and a lack of integrated health records remained challenges, yet preferred networks are beginning to transform the ACO post-acute care landscape.