Ancient agricultural terraces distributed over the mountainous areas in southern China can be taken as new objects for studies on human–environment interaction in the past. However, this type of study is rare. This study selected Longji Terraces (Longsheng, Guilin), one of the famous ancient agricultural terraces, to explore its genesis and paleoenvironmental records. Three soil profiles including an original hillslope woodland soil profile and two terrace profiles were sampled for analyses of physical and geochemical proxies and radiocarbon dating. Cultivated soil in the terrace profile shows aggradation, making the cultivated horizon a bottom–up chronological sedimentary sequence. Two basal samples in the cultivated horizons of two parallel terrace profiles show calibrated ages of 1361–1406 AD and 1335–1384 AD respectively, indicating that the genesis of Longji Terraces occurred during the late Yuan Dynasty in Chinese history. Strong survival pressure and desire for new living space in the turbulent social situation of the late Yuan Dynasty might have been the direct factor forcing some people of national minorities to migrate to the Longji Mountain area, where they effectively utilized the hillslope land and water resources for cultivation by building agricultural terraces and constructing a gravity irrigation network. Multi-proxy analyses of the cultivated horizon of one selected terrace profile reveal that human farming activity has experienced a staged strengthening process, including four stages since the late 14th century. Moreover, some changes of the intensity of human farming activity found within each stage might reflect the impact of climatic fluctuations on human farming activity during the Little Ice Age and the later warming period.