Longitudinal care management (LCM) for high-risk patients is a cornerstone of primary care models aiming to improve quality and reduce costs. Describe the extent to which LCM was implemented in the second year of Comprehensive Primary Care Plus (CPC+), and barriers to and facilitators of implementation. Mixed-methods. Quantitative: 2715 practices participating in CPC+ in 2018. Qualitative: Interviews with practitioners and staff in 23 representative CPC+ practices. Across all CPC+ practices, we report median percentages of empaneled patients placed in the highest-risk tiers and, of those, the median percentage receiving LCM. Across 23 CPC+ practices, we report qualitative findings on LCM implementation. While practices reported benefits of LCM, a small proportion of patients received LCM. Practices placed 2.4% (median) of patients in the highest-risk tier; of these, 30% (median) received LCM. Practices placed 10% (median) of patients in the second-highest-risk tier; of these, 7% (median) received LCM. Interviews revealed LCM uptake across tiers was low because of insufficient care manager staffing. Other challenges included lack of practitioner buy-in to using risk stratification to identify high-risk patients, patients' reluctance to engage in LCM or change behaviors, and limited health information technology functionality for developing, maintaining, and accessing high-risk patients' care plans. Facilitators included embedding care managers within practices and electronic health record functionalities that support LCM. Despite substantial financial and other supports, and practices' perceived benefits of LCM, insufficient care manager staffing and other barriers have limited its potential in CPC+ to date. To expand LCM's reach, practices need additional care managers, training to overcome barriers to patient engagement, better identification of patients who might benefit from LCM, improved information technology tools for risk stratification and care plans, and more practitioner buy-in to risk stratification.