The son of a prominent political figure, Zheng enjoyed early success in the imperial examinations and entered public service at a young age.
After his father's political faction fell from favor, he was blacklisted for years and only returned to office in the 860s, after forming an association with the respected statesman Liu Zhan.
Zheng secured influential court postings during Liu's brief tenure as chancellor, earning a reputation as a talented edict drafter and military strategist.
He remained loyal to Liu when the latter fell from power and was in turn banished from court, but was rehabilitated during the reign of Emperor Xizong and made a chancellor in 874.
Zheng subsequently participated in an unsuccessful military operation to retake the capital and, following a mutiny, rejoined the emperor in Chengdu and was made a chancellor once again.
He was forced into retirement in 883, following disputes with the influential court eunuch Tian Lingzi, and died of illness shortly thereafter.
[5] Zheng Tian himself passed the imperial examinations in the Jinshi class when he was 17, and thereafter served as a staff member under the military governor (Jiedushi) of Xuanwu Circuit (宣武, headquartered in modern Kaifeng, Henan).
Subsequently, Zheng Tian was recalled to the capital Chang'an to serve as Yubu Yuanwailang (虞部員外郎), a low-level official at the ministry of public works (工部, Gongbu).
During the imperial armies' campaign against the rebel Pang Xun, who occupied Xu Prefecture (徐州, in modern Xuzhou, Jiangsu) in 869, Zheng was largely in drafting the numerous edicts involved in the military maneuvers, and it was said that his colleagues admired him for the speed and the elegance with which he wrote.
He was soon promoted to be both deputy minister of census (戶部侍郎, Hubu Shilang) and, after Pang's rebellion was suppressed, chief imperial scholar (翰林學士承旨, Hanlin Xueshi Chengzhi).
[1] In 874, by which time he had become the deputy minister of civil service affairs (吏部侍郎, Libu Shilang), he was again made chief imperial scholar and given the designation Tong Zhongshu Menxia Pingzhangshi (同中書門下平章事), making him a chancellor de facto.
At Zheng's suggestion, the imperial control over the salt and iron monopolies in the region was transferred to Wei He (韋荷) the military governor of Lingnan East, who was authorized to boil water to produce salt for sale, and then use the proceeds to buy food supplies from the prefectures in the southern parts of Jiangxi Circuit (江西, headquartered in modern Nanchang, Jiangxi).
[7] Shortly after Zheng Tian's demotion, however, he was recalled back to Chang'an to serve as the minister of civil service affairs (吏部尚書, Libu Shangshu).
On the way, as he was going through Luo Valley (駱谷, in modern Xi'an, Shaanxi), Zheng intercepted him and begged him not to leave Chang'an's vicinity, requesting that he go to Fengxiang instead.
At that time, many of the imperial guard soldiers could not catch up with Emperor Xizong on his flight to Chengdu, and Zheng summoned them to Fengxiang to have them join his army, spending his own personal wealth to placate them.
Zheng counterattacked with his ally Tang Hongfu (唐弘夫), and they dealt the Qi forces a major defeat at Longwei Slope (龍尾陂, in modern Baoji).
Once he reached Feng Prefecture (鳳州, in modern Baoji), he submitted a report to Emperor Xizong explaining what had occurred and offering to resign.
Emperor Xizong made Li Changyan the military governor of Fengxiang, while making Zheng an advisor to the Crown Prince, with his office at Luoyang.
[9] It was said that despite the imperial government's exile status, Zheng insisted on proper adherence to regulations—such that, by 883, by which time Huang had again abandoned Chang'an and Emperor Xizong was preparing to return to Chang'an, Zheng had offended the powerful eunuch Tian Lingzi and Tian's brother Chen Jingxuan, the military governor of Xichuan Circuit (西川, headquartered in modern Chengdu), as he rebuffed Tian Lingzi's request to have his assistant Wu Yuan (吳圓) promoted and Chen's desire to receive greater honors than chancellors.