This research investigated whether changes in youth internalising problems (e.g. anxiety, depression) as a result of a family group cognitive behavioural (FGCB) preventive intervention for families with a parent with a history of major depression had a 'cascade effect' on youths' social problems over 24 months. Compared to a minimum intervention condition, the FGCB intervention significantly reduced youth internalising problems at 12 months that was then associated with lower levels of social problems at 18 months. The FGCB intervention also reduced internalising problems at 18 months, which was also associated with fewer social problems for youth at 24 months. Overall, the findings show that reductions in youth internalising problems can lead to lower levels of social problems and suggest that targeting internalising problems may be an effective method for reducing the social problems of children with parents who have a history of major depression.