Wei Zheng was born to a poor family in modern Hebei, and joined Li Mi's rebellion against the Sui dynasty in his youth.

Wei Zheng's promotion to this position gave him far broader freedom to criticise others, particularly the emperor, than other officers of the court.

His advice and criticism were not always accepted, but in accordance with Confucian etiquette, the emperor would concede to his suggestions with some regularity.

Wei lost his father early in life and was poor, but had great expectations, not caring about making wealth.

In 617, the secretary general of Wuyang Commandery (武陽, part of modern Handan, Hebei), Yuan Baozang (元寶藏), rebelled against the Sui as well and submitted to Li Mi.

Wei subsequently drafted submissions from Yuan to Li Mi, suggesting that Li Mi attack and seize nearby Wei Commandery (魏郡, also part of modern Handan) and a large food storage that Emperor Yang built, Liyang Storage (黎陽倉, in modern Hebi, Henan).

Pleased, Emperor Taizong stated, "Wei had me send a messenger, and as a result the Lingnan region was pacified, better than if 100,000 men were sent.

Wen nevertheless stated to Emperor Taizong, "Wei did not care about public perception or appearance of conflict of interest.

In 629, Emperor Taizong made Wei the head of the Palace Library, but gave him the additional designation Canyu Chaozheng (參豫朝政), making him a de facto chancellor.

Later that year, when the assistant censor Quan Wanji (權萬紀) accused the chancellors Fang Xuanling and Wang Gui of improper selection of officials, Emperor Taizong initially ordered Hou Junji to investigate.

Wei, however, pointed out that both Fang and Wang were trusted officials who should be given free rein and not have the details of their acts be picked on.

Late in 630, Qu Wentai (麴文泰), the king of Gaochang, was set to arrive at Chang'an to pay homage to Emperor Taizong.

In 631, despite Wei's advice against it, Emperor Taizong began setting up a feudal scheme where the great contributors to Tang rule would be granted posts as prefectural prefects, to be inherited by their descendants.

In 632, at the urgings of many officials who believed that he should go to Mount Tai to pay homage to heaven and earth (known as Fengshan), Emperor Taizong considered doing so, but Wei opposed, and this led to a famous conversation between them, which showcased Wei's personal philosophy against overextending the state: Emperor Taizong asked, "You oppose my conducting of Fengshan.

Wei responded, "Even though Your Imperial Majesty has accomplished these six achievements, we are still inheriting the state left after the great confusion at the end of Sui rule.

When Your Imperial Majesty heads east [to Mount Tai], the thousands of horses and wagons require supplies everywhere they go, and it is difficult for the local government to do so.

Why would Your Imperial Majesty want to simply hold a great Fengshan ceremony but receive real detriments?

"Incidentally, at that time, many prefectures around the Yellow River were suffering floods, so Emperor Taizong gave up the idea.

He sent some of his servants back to Chang'an, and they were staying at the official lodges at Weichuan County (湋川, also in modern Baoji), when the chancellors Li Jing and Wang Gui arrived.

Wei advised against it, pointing out that this was contrary to Emperor Ming of Han's observation that his sons should not be as honored as his brothers.

On another occasion, after Emperor Taizong returned from an imperial gathering, he was angry and yelled, "Let me find a chance to kill this farmer!"

Wei responded, "It was written that Emperor Shun warned his officials, 'Do not obey me in my presence and make secret speeches against what I do.'

In 634, when Emperor Taizong wanted to send officials to visit the circuits to see how the people were doing in the provinces, Li Jing recommended Wei.

In 638, there was yet another incident where Wang, by then the minister of ceremonies, suggested that high-level officials should not have to yield to imperial princes when their processions meet.

In 638, Emperor Taizong, celebrating the birth of a grandson, held a feast for imperial officials, at which he made the comment: Before I took the throne, it was by Fang Xuangling's assistance that I was able to seize power.

In 639, after Ashina Duobi's nephew Ashina Jiesheshuai formed a plot to ambush Emperor Taizong, but his plot was discovered and destroyed, Emperor Taizong created a Tujue prince who had served him faithfully, Li Simo (né Ashina Simo) the Prince of Huaihua, as the khan of a newly recreated Eastern Tujue state (as Qilibi Khan), and had him take the Eastern Tujue people outside of Tang territory, to be settled between Tang and Xueyantuo.

(Emperor Taizong would later regret this in 642, when the need to station soldiers at Gaochang became more and more expensive in monetary and human costs.)

However, Wei pointed out that chancellors were supposed to be responsible for all affairs of state, and Emperor Taizong, realizing that he had erred, was humbled.

When Wei grew better, he personally went to the palace to decline, but Emperor Taizong pointed out the symbolic importance of having someone as important as himself serve as Li Chengqian's advisor, stating that even if Wei could only lie on the bed, his being named a senior advisor to Li Chengqian would help clarify the situation.

Wei Zheng is venerated as a door god in some Chinese and Taoist temples, usually in partnership with Li Shiji.