Background: The use of mediated learning in cognitive training has been shown to be effective in enhancing students' cognitive development. Nonetheless, its effects on language development are less explored. Aims: This study examined the effects of an early cognitive intervention (Think Bright program) in enhancing the cognitive and language development of Hong Kong preschoolers with developmental delay. Method: Sixty-eight children (48 boys and 20 girls; mean age = 58 months) with developmental delay were recruited from preschool rehabilitation centres and randomized to two groups (Think Bright training vs. active control). Each child in the Think Bright group received 12 sessions of 1-hr individual training on thinking skills over 6 months. The control group received the same amount of training based on the regular training regimen adopted at the rehabilitation centres. Results: After a 6-month intervention, the Think Bright group significantly outperformed the control group in language, general cognition, analogical thinking, sequential thinking, and logical reasoning. The Think Bright teachers' mediation skills significantly improved during the course of intervention and correlated moderately with the improvement in students' language abilities. Conclusion: This study has shown promising results on the effectiveness of using mediated learning in early cognitive intervention in enhancing both the cognitive and language development of preschoolers with developmental delay.