We investigated the impact of a subsidized, needs-based preschool program (Study 1; N = 1,894) and California’s universal but age-restricted transitional kindergarten (TK) program (Study 2; N = 1,093) on school readiness. We applied Mahalanobis matching—a quasi-experimental data analysis method used to create equivalent groups—to data from 3 recent kindergarten cohorts in a large, urban school district in San Francisco. We matched students in each preschool group to demographically similar kindergarteners who did not attend public preschool in the district. Students’ literacy, cognitive/fine motor, and social-emotional skills were assessed by kindergarten teachers in the first six weeks of the school year. We observed substantial heterogeneity in program effects only for literacy across the needs-based program ( β = 0.27) and TK ( β = 0.53). For cognitive/fine motor skills, effects were similar across the needs-based program ( β = 0.23) and TK ( β = 0.28). Only TK had a positive effect on social emotional skills ( β = 0.12). Across these two programs, which serve students who go on to enroll in kindergarten classrooms together, we discuss the structural program differences that could explain variation in effectiveness and implications for policy as California expands its TK program to cover all four-year-olds. (author abstract)