It is not known when Du Zhenglun was born, but it is known that he was from what would eventually become Xiang Prefecture (相州, in modern Handan, Hebei).
As Sui dynasty imperial examinations were considered unusually difficult—with only 10-odd examinees passing each year—the Du household received great acclaim for having three brothers who did so.
During the reign of the Tang dynasty's founder, Emperor Gaozu, Du Zhenglun was initially serving as the military advisor to the commandant at Qi Prefecture (齊州, roughly modern Jinan, Shandong).
On one occasion, Emperor Taizong stated, "Everyday, as I sit in judgment of important matter and get ready to speak, I think carefully about whether what I have to say has any benefit or harm to the people.
After Li Chengqian was revealed to have plotted with the general Hou Junji to overthrow Emperor Taizong in 643, Du was accused of having received a gold belt from Hou, and he was stripped of his rank and exiled to Huan Prefecture (驩州, also in modern Vietnam).
In 656, Du Zhenglun was serving as the deputy minister of treasury, when Emperor Taizong's son and successor Emperor Gaozong promoted him to serve as Huangmen Shilang (黃門侍郎), the deputy head of the examination bureau (門下省, Menxia Sheng).
Emperor Gaozong also gave him the designation of Tong Zhongshu Menxia Sanpin (同中書門下三品), making him a chancellor de facto.
In 657, he was made Zhongshu Ling (中書令), the head of the legislative bureau and a post considered one for a chancellor.
On December 6, 658,[2] Emperor Gaozong demoted both to be prefects—in Du's case, the prefect of Heng Prefecture (橫州, roughly modern Nanning, Guangxi).