Life cycle assessment (LCA) is an outstanding methodology used to estimate the environmental impacts resulting from the manufacture of a given product or service, examining each stage of its life cycle, from the extraction of natural resources to its disposal. The purpose of this research is to present a practical application of LCA in optimizing the sustainability of steel–concrete composite footbridges, considering the cost of the construction and its environmental impact as objectives. The established design variables for this include the slab's concrete strength and thickness, the dimensions of the welded steel I-girder, and the interaction degree of the composite beam. The Multi-Objective Harmony Search (MOHS) algorithm is applied to address the optimization problem. LCA studies are applied to estimate the environmental impacts associated with the consumption of concrete and steel by the structure, using CO2 emissions and embodied energy as environmental impact criteria. The results of the multiobjective optimization allow to build associations among different objectives and evaluate viable trade-offs for the project. By applying the estimates of CO2 emissions and embodied energy provided by LCA studies, it was possible to successfully quantify and evaluate the environmental impact of the structure. Initially, a cost-versus-CO2 emission comparison is presented, and among the findings, it was that solutions that perform well in terms of costs are also viable in terms of CO2 emissions. Following that, the carbon dioxide emissions are compared to the structure's embodied energy, also revealing a linear association. The scarcity of infrastructure, specially in developing countries, in addition to the low economic resources, underline the significance of optimization and LCA as techniques to contribute to the sustainable development of structures with lower expenses while resulting in reduced environmental impacts.