There is a growing body of knowledge on the link between environmental management and supply chain management, but there is contradicting evidence on the impact of a proactive environmental strategy on environmental performance. Therefore, this paper investigates the impact of proactive environmental strategy on environmental performance as mediated by environmental investments. We also consider the antecedents of the adoption of proactive environmental strategy. We develop and test hypotheses, using data collected from 96 Turkish manufacturers through an online questionnaire. The model was tested using partial least squares (PLS), a structural equation modelling method. The results show that a proactive environmental strategy leads to higher environmental investments; both internally and externally in collaboration with suppliers. Our findings support our hypothesis that environmental investments act as a mediating variable between proactive environmental strategy and environmental performance. The results also show that customer pressure and, particularly, organisational commitment positively impact the extent to which firms adopt a proactive environmental strategy.