Background: Primary care delivery for multimorbid patients is complex, due to single disease–oriented guidelines, complex care needs, time constraints and the involvement of multiple healthcare professionals. Co-creation of care, based on the quality of communication and relationships between healthcare professionals and patients, may therefore be valuable. This longitudinal study investigates the relationships of co-creation of care to physical and social well-being and satisfaction with care among multimorbid patients in primary care. Methods: In 2017 and 2018, longitudinal surveys were conducted among multimorbid patients from seven primary care practices in Noord-Brabant, the Netherlands (n = 138, age = 73.50 ± 9.99). Paired sample t-tests and multivariate regression analyses were performed. (3) Results: Co-creation of care improved significantly over time (t = 2.25, p = 0.026), as did social well-being (t = 2.31, p = 0.022) and physical well-being (t = 2.72, p = 0.007) but not satisfaction with care (t = 0.18, p = 0.858). Improvements in co-creation of care from T0 to T1 were associated with social well-being (B = 0.157, p = 0.002), physical well-being (B = 0.216, p = 0.000) and satisfaction with care (B = 0.240, p = 0.000). (4) Conclusions: Thus, investment in co-creation of care by primary care practices may lead to better outcomes for multimorbid patients.