As an inorganic–organic composite material, cement and emulsified asphalt (CA) mortar has been used widely in ballastless slab track structures, and its performance is influenced significantly by environmental conditions. In this study, we tested the long-term creep of CA mortar at different temperatures using a self-designed loading device, and explored the mechanisms by which high temperatures influence CA mortar creep based on several microscopic testing methods. The results indicated that high temperatures cause the creep degree and rate of CA mortar to increase significantly. Subsequent to creep at high temperatures, the mechanical properties and microstructure of CA mortar changed remarkably, and asphalt migration may be an important cause of changes in CA mortar at high temperatures. This will be useful for future studies of the evolution of CA mortar performance under long-term service conditions.