He hailed from Chouxian 犨縣 in the commandery of Nanyang 南陽 (today's Lushan 魯山, Henan) and was known for his restrained and sensitive character. During the reign of Emperor Ming 漢明帝 (r. 57-75 CE) he was appointed steward (jisi 給事) in the household of the heir apparent, and later rose to the position of palace attendant (xiao huangmen 小黃門, then Palace Attendant-in-ordinary (zhongchangshi 中常侍).
In the early reign of Emperor He 漢和帝 (r. 88-105 CE) he was made Director of the Office of Imperial Parks Products (goudun ling 鉤盾令). The emperor was too young to rule, and government affairs were therefore regulated by Empress Dowager Dou 竇太后 and her brother Dou Xian 竇憲. All court officials accepted the domination by the Dou family, and just Zheng Zhong crititiced this situation because he was not bound to any faction apart from the emperor. When the power of the Dou family was curtailed, Emperor he later promoted him to the position of Director of the Palace Domestic Service (dachangqiu 大長秋 and gave him the title of Township Marquis of Chaoxiang 鄛鄉侯.
In these years Emperor He used to consider Zheng's suggestion in government affairs. Although Zheng Zhong is not characterized as an evil person, he was the first of the eunuchs who critically influenced court affairs in the second half of the Later Han period. His territory was inherited by his foster son Zheng Hong 鄭閎.