Preschool-age irritability is a transdiagnostic marker of internalizing and externalizing problems. However, researchers have generally been reluctant to examine irritability within a clinically salient framework at younger ages due to some instability during the “terrible twos” period. Developmentally sensitive and dense measurements to capture intra- and inter-individual variability, as well as exploration of developmental processes that predict change, are needed. This study aimed to examine (1) the trajectories of irritability at the transition to toddlerhood (12–24 months of age) using repeated measures, (2) whether effortful control was associated with individual differences in level and growth rate of irritability, and (3) whether individual differences in the irritability trajectories were associated with later psychopathology. Families were recruited when the child was 12–18 months old (N = 333, 45.65% female). Mothers reported on their toddler’s irritability at baseline and every two months until a follow-up laboratory assessment approximately one year later. Effortful control was measured at baseline. Clinical internalizing/externalizing symptoms were measured at the follow-up assessment. Hierarchical linear models revealed an increase in irritability over time, yet there was relatively little within-person variability. Effortful control was only associated with the level of irritability and not growth rate. Level of irritability was associated with internalizing, externalizing, and combined symptoms, but growth rate was not. Findings suggest intraindividual stability in irritability at the transition to toddlerhood and the possibility that screening for elevated irritability at toddler age is meaningful.