Objectives: To explore the role of the built environment, measured by both Objective and subjective data for the neighbourhood, in children’s trajectories of mental health and cognition from the early years until the end of primary school in the general population.   Background: The extant research on this has produced mixed Findings. One reason may be its focus on a single aspect of the built environment. Here, it was measured in a variety of ways, using both Objective and subjective measures of the neighbourhood. We also controlled for indoor home environment and both neighbourhood and family socioeconomic disadvantage.    Methods: We used data from the Millennium Cohort Study, a large general-population cohort study in the UK. The built environment was measured in various ways, including by direct observation of the immediate neighbourhood by interviewers. It was also measured by administrative data on socioeconomic disadvantage and urbanicity as well as Objective data on greenspace and air pollution. We also controlled for the home’s physical and social environment, measured by both parent report and third-party assessments. Child outcomes (mental health and cognition) were measured across ages 3 to 11 years.  Mental health was measured with the Strengths and Difficulties Questionnaire, and cognitive ability with the British Ability Scales. We fitted growth curve models and accounted for confounding.    Results: Neighbourhood disorder, as measured by third-party observation, was associated with emotional symptoms and conduct problems at age 3 years and with the trajectories of cognitive ability from ages 3 to 11 years. These effects were robust to adjustment for confounders. Greenspace, urbanicity and air pollution did not have any effect on our outcomes after adjustment.   Conclusions: Our Findings shed light on the role of the built environment in children’s mental health and cognition, but also on the importance of its appropriate measurement.