Numerous tools have been developed to assist environmental decision-making, but there has been little examination of whether these tools achieve this aim, particularly for urban environments. This study aimed to evaluate the use of the i-Tree Eco tool in Great Britain, an assessment tool developed to support urban forest management. The study employed a documentary review, an online survey, and interviews in six case study areas to examine five impacts (instrumental, conceptual, capacity-building, enduring connectivity, and culture/attitudes towards knowledge exchange) and to identify which factors inhibited or supported achievement of impact. It revealed that the i-Tree Eco projects had helped to increase knowledge of urban forests and awareness of the benefits they provide. While there was often broad use of i-Tree Eco findings in various internal reports, external forums, and discussions of wider policies and plans, direct changes relating to improved urban forest management, increased funding or new tree policies were less frequent. The barriers we identified which limited impact included a lack of project champions, policy drivers and resources, problems with knowledge transfer and exchange, organisational and staff change, and negative views of trees. Overall, i-Tree Eco, similar to other environmental decision-making tools, can help to improve the management of urban trees when planned as one step in a longer process of engagement with stakeholders and development of new management plans and policies. In this first published impact evaluation of multiple i-Tree Eco projects, we identified eight lessons to enhance the impact of future i-Tree Eco projects, transferable to other environmental decision-making tools.