Comprehensive Primary Care Plus (CPC+) was a multipayer payment reform model that provided incentives for primary care practices to lower spending and improve quality performance. Although CPC+ has been evaluated in Medicare, little is known about its impact in the private sector. Using claims and enrollment data from the period 2013–20 from two large insurers in Michigan, we performed difference-in-differences analyses and found that CPC+ was not associated with changes in total spending (−$44.70 per year) or overall quality performance (−0.1 percentage point). These changes did not vary systematically across CPC+ cohorts, tracks, regions, or participation in prior primary care innovations. We conclude that CPC+ did not improve spending or quality for private-plan enrollees in Michigan, even before accounting for payouts to providers. This analysis adds to existing evidence that CPC+ may cost payers money in the short term, without concomitant improvements to care quality.