Objective To assess the effects of past Medicaid eligibility expansions to parents on coverage, access to care, out‐of‐pocket (OOP) spending, and mental health outcomes, and consider implications for the Affordable Care Act (ACA) Medicaid expansion. Data Sources Person‐level data from the National Health Interview Survey (1998–2010) is used to measure insurance coverage and related outcomes for low‐income parents. Using state identifiers available at the National Center for Health Statistics Research Data Center, we attach state Medicaid eligibility thresholds for parents collected from a variety of sources to NHIS observations. Study Design We use changes in the Medicaid eligibility threshold for parents within states over time to identify the effects of changes in eligibility on low‐income parents. Principal Findings We find that expanding Medicaid eligibility increases insurance coverage, reduces unmet needs due to cost and OOP spending, and improves mental health status among low‐income parents. Moreover, our findings suggest that uninsured populations in states not currently participating in the ACA Medicaid expansion would experience even larger improvements in coverage and related outcomes than those in participating states if they chose to expand eligibility. Conclusions The ACA Medicaid expansion has the potential to improve a wide variety of coverage, access, financial, and health outcomes for uninsured parents in states that choose to expand coverage.