Objective To quantify the effect of Medicaid expansions on office‐based care among the newly eligible. Data Source 2008–2014 Medical Expenditure Panel Survey. Study Design The main sample is adults age 26–64 with incomes ≤138% of poverty who were not eligible for Medicaid prior to the Affordable Care Act. For this population, difference‐in‐differences linear probability models compare utilization between expansion and nonexpansion states and between 2008–2013 and 2014. Extraction Methods Medicaid eligibility is simulated using data on family relationships, state of residence, and income. Principal Findings Relative to comparable adults in nonexpansion states, newly eligible adults in expansion states were 9.1 percentage points more likely to have any office‐based primary care physician visit in 2014, a 21.4% increase from 2013 (p‐value = .004); 6.9 percentage points more likely to have a specialist visit, a 25.2% increase from 2013 (p‐value = .036); and 5.1 percentage points more likely to have a visit with a nurse practitioner, nurse, or physician assistant, a 34.5% increase from 2013 (p‐value = .016). Conclusions State Medicaid expansions in 2014 were associated with greater likelihoods of visits with a variety of office‐based providers. The estimated effects are larger among newly eligible compared with previous estimates on broader populations of low‐income adults.