Rates of treatment utilization decline as adolescents make the transition to adulthood even though young adults are particularly vulnerable to the negative outcomes of untreated mental illness. Although a variety of factors have been explored to explain decreased treatment utilization in this age group, previous research has almost exclusively employed cross-sectional methods rather than following a group of youth as they enter adulthood. The current study aims to address this methodological limitation by assessing treatment utilization in emerging adults who began participating in a longitudinal study during childhood. One hundred and thirty seven youth who turned 18 during the 96-month follow-up period were included in the current analyses. Demographic and socioeconomic variables such as sex, race, and insurance status and clinical variables such as psychiatric diagnoses and perceptions of treatment effectiveness were investigated as factors potentially associated with outpatient treatment use before and after age 18. Prior to age 18, youth reported using outpatient services at 75% of their visits, but after age 18, outpatient treatment utilization dropped to around 50%. White race, increased parental stress, and increased parental perception of treatment usefulness were associated with greater treatment use prior to age 18, whereas only increased youth perception of symptom-related dysfunction were associated with increased treatment use after age 18. Findings point to the importance of including youth preferences and perceptions of dysfunction in treatment decisions across adolescence in order to optimize treatment use following the transition to adulthood.