Recent reviews of the rapidly growing scientific literature on neighbourhood green space and health show strong evidence for protective and restorative effects on mental wellbeing. However, multiple informants are common when reporting mental wellbeing in studies of children. Do different informants lead to different results? This study utilised nationally representative data on Goodman’s 25-item Strengths and Difficulties Questionnaire reported by 3083 children (aged 12–13 years old), and their parents and teachers. Multilevel models were used to investigate whether similar associations between child mental wellbeing (as measured using the total difficulties score and the internalising and externalising subscales) and neighbourhood green space quantity and quality are obtained regardless of the informant. After adjustment for confounders, higher green space quantity and quality were associated with consistently more favourable child mental wellbeing on all three measures, regardless of the informant. However, associations with green space quantity were statistically significant (p < 0.05) only for the parent-reported total difficulties score and the internalising subscale. Significant associations with green space quality were consistently observed for both parent- and child-reported outcomes. Teacher-reported outcomes were not significantly associated with green space exposure. Future studies of green space and child health should acknowledge when different informants of outcomes could lead to different conclusions.