Young children with attention-deficit/hyperactivity disorder (ADHD) can have challenging behaviors putting them at risk for preschool expulsion and for adverse outcomes across child development, health, and education. We examined the association of preschool expulsion with ADHD symptoms, diagnosis, treatment, and functioning among children with ADHD. Using the cross-sectional National Survey of the Diagnosis and Treatment of ADHD and Tourette Syndrome on 2947 children aged 5 to 17 years ever diagnosed with ADHD, parents reported on preschool expulsion, ADHD symptoms, diagnosis, treatment, and functioning. Weighted analyses included calculations of estimated means, prevalence, and prevalence ratios. Preschool expulsion was experienced by 4.4% of children ever diagnosed with ADHD (girls: 1.5%; boys: 5.7%). Children with preschool expulsion had lower mean ages at first concern about ADHD symptoms, ADHD diagnosis, and initiation of ADHD medication and had higher prevalence of severe ADHD symptoms and other mental, behavioral, or developmental disorders. A history of preschool expulsion was associated with difficulties with overall school performance, organized activities, writing, handwriting, and the parent-child relationship, but not with math, reading, or peer or sibling relationships. Children with preschool expulsion more often received school supports, behavioral classroom management, peer intervention, and social skills training. Among children ever diagnosed with ADHD, history of preschool expulsion was associated with more severe ADHD symptoms, other disorders, earlier diagnosis and medication initiation, and academic and social impairment. Health care providers can use preschool expulsion as an indicator of risk for children with ADHD and connect families to effective treatments.