Previous studies have demonstrated that sluggish cognitive tempo (SCT) behaviors are empirically distinct from inattentive (IN) behaviors that are used to define attention-deficit/hyperactivity disorder. However, most studies used cross-sectional designs during middle childhood. Using parent and teacher ratings from the Family Life Project (N = 1,173), we investigated the factor structure, longitudinal measurement invariance, developmental trajectories, and predictors of developmental change in SCT and IN from age 3 years through Grade 5. SCT and IN were dissociable but correlated constructs that exhibited longitudinal invariance for both informants. Mean levels of SCT increased modestly with age, becoming more prominent between age 5 years and first grade, while IN was more stable. Lower parental education was associated with higher parent- and teacher-reported SCT, male sex was associated with higher teacher-reported IN, and African American race was associated with higher teacher-reported IN but lower teacher-reported SCT. These findings support the validity of SCT starting in early childhood.