<p>The inherently heterogeneous nature of concrete dictates its age-dependent mechanical properties, manifesting in a gradual acquisition of strength that is intricately linked to the progression of hydration reactions and the resultant products, as well as the prevailing environmental conditions. In reinforced concrete (RC) structures, the evolution of concrete’s mechanical properties during the construction phase significantly influences both the safety and long-term performance of these structures. In general, multiple effects induced by the various factors on the development of concrete strength co-occur, the external influences may have positive or negative implications on concrete strength. However, the investigations on the development mechanism of early-age concrete strength under specific influences, including sustained loading, elevated temperature, and freeze-thawing, have been reported, but are still insufficient. Thus, to promote the accuracy of the safety evaluation and optimize the structural design and construction of new-built concrete structures subjected to complex external environments, the research progress on the external factors influencing early-age mechanical properties of concrete is reviewed worldwide. This paper delivers an overview of research on the environment-related mechanisms of early-age concrete. The review results help enhance the understanding of the specific development of such behavior on the service-life performance of concrete structures under exceptional circumstances after concrete cast.</p>