In 2010, the province of Ontario introduced a new universal two-year play-based full-day kindergarten program. The authors exploited the phasing-in of this program over five years, allowing a natural experiment in which children from full-day kindergarten could be compared with those from half-day kindergarten in matched neighborhoods. Children (N = 592) were followed from kindergarten to Grade 2 with direct learning and self-regulation measures. Grade 3 wide-scale achievement test scores were available for 269 of the children. Results showed lasting benefits of full-day kindergarten on children’s self-regulation, reading, writing, and number knowledge to the end of Grade 2, including some benefits for vocabulary. Full-day kindergarten children were significantly more likely to meet provincial expectations for reading in Grade 3. The study points to the benefits of a play-based full-day kindergarten program and brings evidence to bear on the mixed findings in the research literature about the fade-out effects of full-day kindergarten.