This study examined the influence of Head Start duration on teacher‐reported children's approaches to learning, behavioral problems, and cooperative classroom behaviors at the end of kindergarten. Propensity score matching was used to create comparable samples of children who experienced different durations of Head Start. Analysis of the Head Start Family and Child Experiences Survey showed that children who attended 2 years of Head Start showed a higher level of approaches to learning (effect size [ES] = .53), cooperative classroom behaviors (ES = .35), and fewer problematic behaviors (ES = ‐.43) in kindergarten. The effects of 2 years of attendance of Head Start were most prominent for children raised in families with high‐risk factors and for Black children, particularly with improvement in approaches to learning. This finding supports the argument that a longer exposure from an earlier age to a preschool program may contribute to improving school readiness for children from economically disadvantaged families.