Medicare finances health care for most US patients with end-stage kidney disease (ESKD), regardless of age. The 2011 Medicare prospective payment system (PPS) for dialysis reduced reimbursement for hemodialysis, and the 2014 Patient Protection and Affordable Care Act (ACA) Marketplace increased patient access to new private insurance options, potentially influencing organizations that provide health care, such as hospitals, nursing homes, and dialysis facilities, to adjust their payer mix away from Medicare sources. To describe Medicare enrollment trends among patients with incident ESKD in 2006 to 2016. This retrospective cohort study involved US patients aged 18 to 64 years who were not enrolled in Medicare at dialysis initiation in 2006 to 2016, with 1-year follow-up through 2017. Data analysis was conducted April 2021 to June 2022. The exposure of interest was a 3-category indicator of time, whether patients initiated dialysis before policies were enacted (2006-2010), in the first years of the Medicare ESKD PPS (2011-2013), or during the Medicare ESKD PPS and implementation of the ACA Marketplace (2014-2016). Patient-level Medicare enrollment through the first year of dialysis. Logistic regression and Cox models were used to examine associations of time, patient characteristics, and Medicare enrollment, adjusting for patient demographic, clinical, and market-level characteristics. Of 335 157 patients aged 18 to 64 years with ESKD not actively enrolled in Medicare when they initiated dialysis in 2006 to 2016, the mean (SD) age was 49.9 (10.8) years, 198 164 (59.1%) were men, 188 290 (56.2%) were White, and 313 622 (93.6%) received in-center hemodialysis. New Medicare enrollment was higher in 2006 to 2010 (110 582 patients [73.1%]) than after the Medicare ESKD PPS and ACA Marketplace in 2014 to 2016 (55 382 patients [58.5%]). In adjusted analyses, declining Medicare enrollment was associated with implementation of 2011 Medicare ESKD PPS and 2014 ACA policies and was disproportionately lower among younger, racially minoritized, and ethnically Hispanic patients. There was declining Medicare enrollment among new dialysis patients associated with the 2011 Medicare ESKD PPS and 2014 ACA Marketplace that raise concerns about benefits and harms to patients and payers and continued disparities in kidney care. As the dialysis payer mix moves toward higher proportions of patients not covered by Medicare, it will be important to understand the implications for health care system and patient outcomes.