Cao Teng served as an Attendant at the Yellow Gates (小黃門) in the Han imperial court during the reign of Emperor An.
In 126, after Emperor Shun ascended the throne, Cao Teng was promoted to Central Regular Attendant (中常侍), the highest possible rank for a court eunuch.
Despite his young age, Liu Zuan showed signs of resentment towards Liang Ji's domineering influence, and was soon poisoned by the general.
After Liu Zhi, or Emperor Huan as he came to be known, ascended the throne, Cao Teng was rewarded with enfeoffment as the Marquis of Fei Village (費亭侯) for his participation in the succession debate.
The people he promoted were all very famous throughout the empire, such as Yu Fang (虞放), Bian Shao (邊韶) and Zhao Dian (趙典).