Background Sluggish cognitive tempo (SCT) is distinct from attention‐deficit/hyperactivity disorder inattention (ADHD‐IN) and concurrently associated with a range of impairment domains. However, few longitudinal studies have examined SCT as a longitudinal predictor of adjustment. Studies to date have all used a relatively short longitudinal time span (6 months to 2 years) and only rating scale measures of adjustment. Using a prospective, multi‐method design, this study examined whether SCT and ADHD‐IN were differentially associated with functioning over a 10‐year period between preschool and the end of ninth grade. Methods Latent state‐trait modeling determined the trait variance (i.e. consistency across occasions) of SCT and ADHD‐IN across four measurement points (preschool and the end of kindergarten, first grade, and second grade) in a large population‐based longitudinal sample (N = 976). Regression analyses were used to examine trait SCT and ADHD‐IN factors in early childhood as predictors of functioning at the end of ninth grade (i.e. parent ratings of psychopathology and social/academic functioning, reading and mathematics academic achievement scores, processing speed and working memory). Results Both SCT and ADHD‐IN contained more trait variance (Ms = 65% and 61%, respectively) than occasion‐specific variance (Ms = 35% and 39%) in early childhood, with trait variance increasing as children progressed from preschool through early elementary school. In regression analyses: (a) SCT significantly predicted greater withdrawal and anxiety/depression whereas ADHD‐IN did not uniquely predict these internalizing domains; (b) ADHD‐IN uniquely predicted more externalizing behaviors whereas SCT uniquely predicted fewer externalizing behaviors; (c) SCT uniquely predicted shyness whereas both SCT and ADHD‐IN uniquely predicted global social difficulties; and (d) ADHD‐IN uniquely predicted poorer math achievement and slower processing speed whereas SCT more consistently predicted poorer reading achievement. Conclusions Findings of this study – from the longest prospective sample to date – provide the clearest evidence yet that SCT and ADHD‐IN often differ when it comes to the functional outcomes they predict.