Since the 18th National Congress of the Communist Party of China, China has made systematic progress in ecological civilization construction. Notable achievements in areas such as environmental governance and the use of renewable energy have driven continuous improvement in ecological welfare performance. Against this backdrop, low-carbon city pilot projects, as an important regulatory innovation, aim to promote coordinated development between the economy, society, and the ecological environment by advancing low-carbon and energy-saving technologies and improving resource utilization efficiency. This aligns with the core orientation of ecological welfare performance, which focuses on people and the comprehensive enhancement of multidimensional well-being. However, existing research lacks a systematic analysis of how low-carbon city construction affects ecological welfare performance, and its effects in terms of net outcomes, transmission mechanisms, and urban heterogeneity remain unclear. Therefore, in-depth research on this issue not only helps reveal the pathways through which policy drives the improvement of ecological well-being but also provides theoretical and empirical references for optimizing urban low-carbon governance systems. This paper constructs an evaluation system for urban ecological welfare performance. Based on input and output dimensions, it employs a super-efficiency SBM model to measure the ecological welfare performance of 274 prefecture-level and above cities nationwide from 2008 to 2022. In addition, a multi-period difference-in-differences (DID) model is used to empirically examine the impact and mechanisms of low-carbon city construction on ecological welfare performance. The findings reveal that low-carbon city construction significantly enhances ecological welfare performance. This conclusion remains robust after a series of tests, including parallel trend tests and placebo tests. Heterogeneity analysis indicates that low-carbon city construction exerts a more pronounced enhancement effect on ecological welfare performance in cities southeast of the Hu Line, old industrial base cities, and cities with lower levels of industrial structure upgrading. Mechanism analysis shows that green technology innovation, energy intensity optimization, and public transportation development constitute three key pathways through which low-carbon city construction influences ecological welfare performance. Further analysis uncovers a significant positive spatial spillover effect of low-carbon city construction on ecological welfare performance. Compared with previous studies, the potential marginal contributions of this paper are threefold. First, using low-carbon city pilot policies as a quasi-natural experiment, this paper empirically examines the impact and spatial spillover effects of low-carbon city construction on ecological welfare performance, thereby deepening the theoretical understanding of the connotation of urban sustainable development. Second, this paper investigates heterogeneous factors such as urban geographic location, industrial development foundation, and industrial structure, and elucidates how these factors influence policy outcomes. This provides empirical support for the precise design and differentiated implementation of low-carbon city policies, thereby further expanding the scope of the research. Third, this paper identifies the underlying mechanisms through which low-carbon city construction affects ecological welfare performance from three dimensions: green technology innovation, energy consumption intensity, and public transportation development, offering valuable insights for related research fields. The findings of this study provide a reference for further advancing low-carbon city construction and broadening the pathways to enhance urban ecological welfare performance. Furthermore, deepening research on the relationship between low-carbon city construction and ecological welfare performance helps refine the theoretical framework of urban sustainable development. It also offers practical guidance for optimizing policy design and achieving the goal of delivering ecological benefits to the people.