It is not known when Zheng Lang was born. He came from a prominent family, as both his father
Zheng Xunyu and brother
Zheng Tan served as
chancellors during their respective careers. Zheng Lang himself passed the
imperial examinations in the
Jinshi class in 821, during the reign of
Emperor Muzong, by which time Zheng Xunyu was deceased but Zheng Tan was already a prominent official—but his passage, as well as the passages of several other examinees related to prominent officials, including
Pei Du's son Pei Zhuan (),
Li Zongmin's son-in-law Su Chao (), and Yang Yinshi () the brother of Yang Rushi (), who was one of the lead examiners, were embroiled in controversy as the officials
Duan Wenchang,
Li Deyu,
Yuan Zhen, and
Li Shen, accused the examiners Yang Rushi and Qian Hui () of being unfair in their decisions. As a result, Emperor Muzong ordered a reexamination, while demoting Qian, Li Zongmin, and Yang Rushi, as well as the 10 examinees who passed based on perceived influence. This incident was considered to have precipitated the subsequent decades-long
Niu-Li Factional Struggles. As a result of this incident, Zheng Lang was not able to remain at
Chang'an to start his official service, but instead served as a staff member of the military governor (
jiedushi) Liu Gongchuo () at Shannan East Circuit (, headquartered in modern
Xiangfan,
Hubei). Zheng was eventually recalled to Chang'an to serve as
You Shiyi (), a low-level advisory official at the legislative bureau of government (,
Zhongshu Sheng). == During Emperor Wenzong's reign ==