Foster children are at risk for developmental challenges in several domains, such as cognitive functioning. The aim of the present study was therefore to investigate cognitive functioning at age 8 years among 39 foster children (FC) compared to 36 children in a non-foster comparison group (NFC). Furthermore, to investigate possible predictors of early functioning on FC’s functioning at age 8 years, in addition to possible different developmental trajectories within and between the two groups. Results revealed that FC performed below the mean on all WISC-IV index scores as well as Full Scale Intelligent Quotient and General Ability Index, and their performances were significantly below those of the NFC. Although most Mullen scores at 2 and 3 years of age predicted functioning at age 8 years, receptive language at age 3 and Mullen Early Learning Composite at 2 as well as 3 years of age best predicted functioning at age 8 years. Finally, the foster children showed a small catch-up in cognitive functioning over the 6-years-period. The results add to previous research on foster children’s struggle with cognitive functioning and the longitudinal design highlights the need of early intervention to optimize foster children’s functioning in the long run.