From 1932 to 1936, he worked as a lecturer of historical geography and history at several universities in Peiping, including Yenching, Fu Jen Catholic University, Peking University, and Tsinghua University. In the 1930s, Gu and Tan cofounded the Yugong Society and the journal Chinese Historical Geography. In 1940, Tan joined Zhejiang University and then relocated to inland Guizhou province due to the Japanese invasion of China, as an associate professor. In 1946, after the surrender of Japan, Tan moved to Hangzhou with Zhejiang University, which was moved back to its original campus. From 1947 to 1949, he taught at both Zhejiang University and his alma mater Jinan University in Shanghai. After the founding of the People's Republic of China in 1949, Jinan University was closed, and most of its departments were merged into Fudan University. Tan became a history professor at Fudan, where he stayed for the rest of his career. He was the head of the Department of History of Fudan starting in 1957 and became an academician of the Chinese Academy of Sciences in 1981. From 1982 to 1986, he served as director of the Institute of Historical Geography of China at Fudan University.
His most important work is The Historical Atlas of China, which is also regarded as the most significant work in the field of historical geography in China. He began working on the atlas in 1955 and partly based it on the work of Qing dynasty scholar Yang Shoujing. It took him more than 30 years to complete the work, and the eight-volume atlas was published from 1982 to 1988. The atlas consists of 304 maps covering the thousands of years of Chinese history until the Qing Dynasty and includes over 70,000 place names. The atlas contains many maps that are politically sensitive, especially on Tibet and Xinjiang. According to his student Ge Jianxiong, Tan was under strong political pressure to make the historical Tibet appear smaller, but he insisted on being faithful to history and included Xinjiang as Tibetan territory during the late 8th century, and his version was eventually approved by China's then-reformist leader, Hu Yaobang.
Students
His students include Wang Zhongshu, who studied history with him at Zhejiang University and later became one of the most prominent archaeologists in Asia. At Fudan University, students Zhou Zhenhe and Ge Jianxiong, now well-known historians, were the first two recipients of the doctoral degree in humanities in the People's Republic of China.
