Abstract: Focusing on the national strategy of integrated development in the Yangtze River Delta to build a highland for green development, it is essential to promote the coordinated advancement of high-level ecological protection and high-quality economic development. Therefore, this study established a Green and Low-Carbon Development Index (GLCDI) system for the Yangtze River Delta region within the context of integrated development. This system comprises 29 indicators across six dimensions: green and low-carbon economy, green and low-carbon environment, green energy saving and carbon reduction, green and low-carbon lifestyle, green and low-carbon innovation, and green and low-carbon policies. Using the expert consultation method and the game theory combination weighting method, this paper determined the combined weights, calculated the GLCDI scores of 41 cities of this region in 2015, 2018 and 2021 through a bottom-up weighted sum, and compared the status of green and low-carbon development in the Yangtze River Delta region before and after the implementation of the integration strategy. The research findings are as follows: (1) The average GLCDI score in the Yangtze River Delta region shows an upward trend, with a marked acceleration in green and low-carbon development after 2018, indicating that the integration strategy has significantly boosted green and low-carbon development in this region. At the city scale, Shanghai, Suzhou, and Hangzhou exhibit the best overall performance in green and low-carbon development. (2) The indices for the green and low-carbon economy, environmental, resource, innovation, and policy in the Yangtze River Delta region all show a continuous upward trend. The green and low-carbon lifestyle index initially increases and then slightly decline, but its 2021 score remains higher than in 2015. (3) The implementation of the integrated development policy in the Yangtze River Delta region has a more pronounced promoting effect on the green and low-carbon development of mega-cities, medium-sized and large cities. Overall, the 41 cities in the Yangtze River Delta region exhibit uneven levels of green and low-carbon development, with significant regional imbalances still prominent. The conclusions of this study provide a scientific basis for the continuous promotion of green and low-carbon development in the Yangtze River Delta.