Importance: Medicaid expansions as part of the Patient Protection and Affordable Care Act (ACA) are associated with decreases in the percentage of uninsured patients who have received a new diagnosis of cancer. Little is known about the association of Medicaid expansions with stage at diagnosis and time to treatment initiation (TTI) for patients with head and neck squamous cell carcinoma (HNSCC)., Objective: To determine the association of Medicaid expansions as part of the ACA with stage at diagnosis and TTI for patients with HNSCC., Design, Setting, and Participants: A retrospective cohort study was conducted at Commission on Cancer-accredited facilities among 90 789 patients identified from the National Cancer Database aged 18 to 64 years with HNSCC that was diagnosed during the period from January 1, 2010, to December 31, 2016. Statistical analysis was conducted from February 18 to November 8, 2019., Main Outcomes and Measures: Outcome measures included health insurance coverage, stage at diagnosis, and TTI. Absolute percentage change in health insurance coverage, crude and adjusted difference in differences (DD) in absolute percentage change in coverage, stage at diagnosis, and TTI before (2010-2013) and after (2014-2016) ACA implementation were calculated for Medicaid expansion and nonexpansion states., Results: Of the 90 789 nonelderly adults with newly diagnosed HNSCC (mean [SD] age, 54.7 [7.0] years), 70 907 (78.1%) were men, 72 911 (80.3%) were non-Hispanic white, 52 142 (57.4%) were between 55 and 64 years of age, and 54 940 (60.5%) resided in states with an ACA Medicaid expansion. Compared with nonexpansion states, the percentage of patients with HNSCC with Medicaid increased more in expansion states after the implementation of the ACA (adjusted DD, 4.6 percentage points [95% CI, 3.7-5.4 percentage points]). The percentage of patients with localized disease (American Joint Committee on Cancer stage I-II) at diagnosis increased in expansion states compared with nonexpansion states for the overall cohort (adjusted DD, 2.3 percentage points [95% CI, 1.1-3.5 percentage points]) and for the subset of patients with nonoropharyngeal HNSCC (adjusted DD, 3.4 percentage points [95% CI, 1.5-5.2 percentage points]). The mean TTI did not differ between expansion and nonexpansion states for the cohort (adjusted DD, -12.7 percentage points [95% CI, -27.4 to 4.2 percentage points]) but improved for patients with nonoropharyngeal HNSCC (adjusted DD, -26.5 percentage points [95% CI, -49.6 to -3.4 percentage points])., Conclusions and Relevance: This study suggests that Medicaid expansions were associated with a greater increase in the percentage of patients with HNSCC with Medicaid coverage, an increase in the percentage of patients with localized disease at diagnosis for the overall cohort of patients with HNSCC, and improved TTI for patients with nonoropharyngeal HNSCC.