Twenty-five years after the North American Free Trade Agreement (NAFTA) came into force, what impact has it had on the US-Mexico border environment? This paper asks what lessons NAFTA offers to contemporary debates about trade and environmental governance through analysis of time series data and expert perceptions on the environment of the US-Mexico borderlands. In the early 1990s, scholars and activists argued that NAFTA would have mostly negative impacts on the US-Mexico border, creating water scarcity and increasing air, land and water pollution; degrading ecosystems; and causing health problems. The debate over NAFTA was part of the larger discussion on the environmental impacts of trade and globalization. In response to these concerns, several governance institutions were created to monitor the environment (the Commission for Environmental Cooperation) and to certify and fund improvements to environmental infrastructure along the US-Mexico border (the Border Environment Cooperation Commission and North American Development Bank). NAFTA was recently replaced with a new trade deal (the USMCA). In this paper we review trends in environmental conditions over the past 25 years, mostly on the Mexican side of the border where greater impacts were anticipated, through datasets, institutional reports and scholarly literature. We also present and discuss the results of semi-structured interviews and surveys with 49 experts (researchers, activists, government personnel, and other border institutional actors) to understand the varied legacy of NAFTA on the border environment at 25 years. Although missing data and challenges in attribution complicated our analysis, we found both positive and negative trends in environmental indicators and the literature.