Coverage and access have improved under the Affordable Care Act, yet it is unclear whether recent gains have reached those regions within states that were most in need of improved access to care. We examined geographic variation in Medicaid acceptance among Michigan primary care practices before and after Medicaid expansion in the state, using data from a simulated patient study of primary care practices. We used logistic regression analysis with time indicators to assess regional changes in Medicaid acceptance over time. Geographic regions with lower baseline (<50%) Medicaid acceptance had significant increases in Medicaid acceptance at 4 and 8 months post-expansion, while regions with higher baseline (≥50%) Medicaid acceptance did not experience significant changes in Medicaid acceptance. As state Medicaid expansions continue to be implemented across the country, policy makers should consider the local dynamics of incentives for provider participation in Medicaid.