He was known for his military prowess, particularly displayed in the conquest of Baekje, as well as political skills that allowed him to maintain good relationships with colleagues and the strong-willed Empress Wu.
The chancellor Wei Zheng, who happened to be present, spoke on Liu's behalf, arguing that a reason for Sui's downfall was that the people had no respect for local officials and did not hesitate from resisting them.
Emperor Taizong withdrew his order to execute Liu and further promoted him to be the secretary general of Yueyang County (櫟陽, in modern Xi'an, Shaanxi), close to the capital.
In 659, Liu was made the prefect of Qing Prefecture (青州, roughly modern Weifang, Shandong), at Li's instigation.
Liu Rengui was pleased at the news, and was said to have immediately leapt up and stated, "Heaven is giving glory and wealth to this old man!"
The soldiers largely wanted to accept these orders, but Liu Rengui pointed out if they abandoned Sabi, Baekje would be reestablished and would not be able to be defeated.
Liu Rengui caught them by surprise and captured a number of cities they held, including a strip of territory that allowed Silla forces to supply Sabi with food.
It was said that Liu Rengui was effective in comforting the Baekje people and restoring social order after years of warfare had rendered the region in waste.
In 664, Liu Rengui, by now carrying the higher rank of commandant, submitted a petition pointing out that the soldiers stationed at Baekje was facing much difficulties, after having been away from home for five years, and should be greatly rewarded.
However, the following year (665), as Emperor Gaozong with Empress Wu was preparing for a grand ceremony to make sacrifices to Heaven and Earth at Mount Tai, Liu Rengui returned to Tang along with emissaries from Silla, Tamna, Japan, and the former Baekje, to attend to Emperor Gaozong and Empress Wu during the ceremony.
He also gave Liu Rengui the designation Zhi Zhengshi (知政事), making him a de facto chancellor at the suggestion of Empress Wu.
In 666, Liu was promoted to You Xiang (右相) — the head of Xi Tai (西臺), the legislative bureau of the government and a post considered one for a chancellor.
However, it was at this time that Liu carried out some actions with regard to an old nemesis, Yuan Yishi—who had earlier tried to force him to commit suicide at Li Yifu's direction—that would bring both praises and comments of his being insincere.
This brought whispers from many officials, and when Liu heard that people were commenting about it, he further promoted Yuan to the post of chief treasurer at the ministry of finance.
Later that year, Liu and another high-level official, Dai Zhide, were made Puye (僕射), co-heads of the important executive bureau of government (尚書省, Shangshu Sheng).
In 677, facing the increasing threat from Tufan, Emperor Gaozong had Liu command the army stationed at Taohe Base (洮河軍, in modern Haidong Prefecture, Qinghai).
Later that year, Liu resigned his post as head of the executive bureau, but Emperor Gaozong had him remain as chancellor de facto.
In 682, as the Chang'an region was suffering from a famine, Emperor Gaozong and Empress Wu, in order to reduce the pressure on the imperial food supply, took up residence at the eastern capital Luoyang.
He left Li Zhe in charge of Chang'an, assisted by Liu and fellow chancellors Pei Yan and Xue Yuanchao.
After Emperor Zhongzong's ascension, Liu Rengui was again made Pushe, and he continued to be in charge at Chang'an, formally assisting Li Chongzhao.
Liu thereafter wrote a petition and gave it to Jiang, asking him to deliver it to Empress Dowager Wu when he returned to Luoyang.