--- - |2+ Abstract Low levels of inhibitory control (IC) are associated with multiple cognitive and socio‐emotional difficulties including externalizing behavioural problems and attention deficit hyperactivity disorder (ADHD). However, conclusions depend somewhat on the method of IC assessment. This investigation examined associations between child IC and behavioural problems and maternal depression and anxiety employing a family study design. Child IC was assessed using a parent rating scale, three independent observer ratings, two laboratory‐based temperament episodes, and an executive functioning task. Correlational results showed that parent‐rated IC was associated with all behavioural problems' scales, whereas observed IC was specifically associated with ADHD, aggressive, attention, and externalizing behavioural problems. Structural equation modelling analyses including child age and gender indicated that all IC variables significantly predicted externalizing and ADHD behavioural problems. The model accounted for overlapping variance across parent‐rated IC, child behavioural problems, and maternal mental health symptoms and showed that observed IC predicted externalizing and ADHD behavioural problems over and above these parent–rater effects. Results also showed that maternal depression and anxiety symptoms significantly predicted these behavioural problems even after controlling for the parent‐rater effects. These findings indicate that assessment methods are an important consideration when interpreting the results of early IC investigations. Highlights Do different measures of inhibitory control (IC) predict behaviour problems during preschool? SEM analyses indicated that observed IC predicted externalizing and ADHD behaviour problems over and above parent‐rater effects. Observations of IC show more specificity to externalizing and ADHD problems and are not susceptible to parent‐rater effects. - Infant and Child Development, EarlyView.