Population-based data on educational and employment outcomes in adulthood among individuals diagnosed with autism spectrum disorder (ASD) in childhood are currently limited. To evaluate educational attainment and employment among individuals with and without a diagnosis of ASD before age 12 years in Denmark. This nationwide cross-sectional prevalence study was conducted using data from Danish registers. Individuals with a diagnosis of ASD recorded before age 12 years were identified among all individuals born in Denmark between January 1, 1989, and December 31, 1991, who were alive at age 25 years. Individuals with ASD were then matched on a 10:1 ratio by age, sex, and region of residence with a comparison population of individuals without a diagnosis of ASD at age 12 years. Data were analyzed from March 2019 to December 2020. Autism spectrum disorder diagnosis and diagnostic subtype recorded before age 12 years. Adjusted prevalence ratios (aPRs) with 95% CIs for the completion of compulsory primary and lower secondary school (grade 9), upper secondary school (grades 10-12 or vocational), and tertiary school (university) and for employment by age 25 years were estimated using log-binomial regression analysis. A total of 810 individuals with a diagnosis of ASD before age 12 years were matched with a comparison population of 8100 individuals without ASD. The prevalence of ninth-grade completion was similar among those with and without ASD (785 individuals [96.9%] and 7982 individuals [98.5%], respectively; aPR, 0.98; 95% CI, 0.97-1.00). Compared with those without ASD, persons with ASD had a lower prevalence of completing upper secondary school (6338 individuals [78.2%] vs 286 individuals [35.3%], respectively; aPR, 0.46; 95% CI, 0.42-0.50) and tertiary school (2185 individuals [27.0%] vs 70 individuals [8.6%]; aPR, 0.33; 95% CI, 0.26-0.41) and obtaining employment (4284 individuals [77.7%] vs 177 individuals [27.2%]; aPR, 0.35; 95% CI, 0.31-0.40) at age 25 years. A ninth-grade final examination score was available for 394 individuals (48.6%) with ASD and 7417 individuals (91.6%) without ASD. In an analysis stratified by ASD subtype, individuals diagnosed with childhood autism had lower educational attainment and employment than those diagnosed with Asperger syndrome or pervasive developmental disorder not otherwise specified. A total of 461 individuals (56.9%) with ASD were receiving public assistance or a pension (ie, disability benefits) at age 25 years compared with 1094 individuals (13.5%) without ASD in the comparison population. In this population-based cross-sectional study, a diagnosis of ASD in childhood was not associated with the completion of compulsory primary and lower secondary education (ninth grade). An ASD diagnosis before age 12 years was associated with a lower prevalence of attaining education beyond ninth grade and obtaining employment by age 25 years, indicating a substantially higher risk of reliance on public assistance in young adulthood.