There are socioeconomic-status (SES) differences in cognitive development. Various factors have been proposed that might explain this association, and one of these factors is the home environment. The present study examined a chaotic home atmosphere as a potential mediator of the association between parental SES and cognitive development. A nationally representative sample of children in the United Kingdom was studied when children were 3 years (n = 15,590), 5 years (n = 13,802), and 7 years old (n = 12,661). At each wave, the children completed multiple cognitive tests, and parents provided information on their SES (income, education, and occupation) and the home atmosphere. Mediation effects were tested with longitudinal structural equation modeling. Direct relations between parental SES and cognitive ability were partly mediated by the home atmosphere. The proportion of mediation was 16% for the change in cognitive ability predicted by parental SES. This study suggests that a chaotic home atmosphere might partly explain the association between parental SES and cognitive development.