It is not known when Cui Xuan was born. He was from the "Elder Boling branch" of the prominent Cui clan of Boling, which claimed ancestry from the ruling Jiang house of the Spring and Autumn period state Qi. Cui Xuan's traceable ancestors included officials of Qin Dynasty, Han Dynasty, Jin Dynasty, Northern Wei, and Tang Dynasty. His grandfather Cui Jing served as a secretary general of the executive bureau of government. His father Cui Yuanlüe was a prominent official during the reigns of Emperor Xianzong, Emperor Xianzong's son Emperor Muzong, and Emperor Muzong's sons Emperor Jingzong and Emperor Wenzong. Cui Yuanlüe was considered a potential chancellor at times, but was never chancellor. Cui Yuanlüe's younger brotherCui Yuanshi served briefly as chancellor between the two terms that Cui Xuan would eventually serve as chancellor. Cui Xuan had at least one younger brother, Cui Zi. Cui Xuan himself passed the imperial examinations in the Jinshi class, and thereafter served three terms on the staffs of regional governors, including, at one point, under the former chancellor Li Shi, the military governor of Jingnan Circuit.
During Emperor Wuzong's reign
Early in the Huichang era of Emperor Wenzong's brother and successor Emperor Wuzong, Cui Xuan was recalled to the capital Chang'an to serve as Zuo Shiyi, a low-level advisory official at the examination bureau of government. He was later made Sixun Yuanwailang, a low-level official at the ministry of civil service affairs as well as imperial scholar. He was subsequently promoted to Zhongshu Sheren, a mid-level official at the legislative bureau, as well as chief imperial scholar. Emperor Wuzong favored polo and wrestling, and when Cui advised him against devotion to games, Emperor Wuzong agreed. In 843, he thus made Cui Zhongshu Shilang, the deputy head of the legislative bureau, as well as chancellor de facto with the designation Tong Zhongshu Menxia Pingzhangshi. It was said that, contrary to the previous years, when emperors felt compelled to discuss the commissions of chancellors with powerful eunuchs, Cui's commission was made suddenly, with Emperor Wuzong only having informed the imperial scholar Wei Cong to have Wei draft the edict, without the prior knowledge by the other chancellors or the eunuch directors of palace communications Liu Xingshen or Yang Qinyi. However, it was said that the lead chancellor Li Deyu was jealous of Cui. In 845, he was thus removed from chancellor post and made the minister of census. He was subsequently sent out of Chang'an to serve as the governor of Shanguo Circuit.
During Emperor Xuānzong's reign
Emperor Wuzong died in 846 and was succeeded by his uncle Emperor Xuānzong. Cui Xuan was then made the military governor of Hezhong Circuit as well as the mayor of its capital Hezhong Municipality; he was also created the Viscount of Boling. He was thereafter recalled to Chang'an to serve as chief imperial censor. In 849, Emperor Xuānzong again made him chancellor with the designation Tong Zhongshu Menxia Pingzhangshi as well as Zhongshu Shilang. He eventually came to carry the titles of Zuo Pushe, Menxia Shilang, administrator of Daqing Palace, chief scholar at Hongwen Pavilian, and the Duke of Boling. In 851, with Emperor Xuānzong tired of dealing with repeated Dangxiang rebellions, Cui suggested that a paramount official be appointed to oversee the Dangxiang affairs. Emperor Xuānzong thus made Cui's colleague Bai Minzhong the supreme commander of the forces facing the Dangxiang. In 852, with agrarian rebels occupying Mount Ji, Emperor Xuānzong dispatched the general Wang Zhihong to the region to deal with them. Cui suggested that instead an imperial official be sent to persuade them to submit, so Emperor Xuānzong sent the official Liu Tong, who was able to persuade the Mount Ji rebels to surrender. In 855, with Huainan Circuit suffering from a famine, but the military governor Du Cong not governing the circuit diligently, Emperor Xuānzong removed Du from the post and sent Cui out to Huainan to serve as its military governor, continuing to carry the Tong Zhongshu Menxia Pingzhangshi title as an honorary title. He was also created the greater title of Duke of Wei. As he departed for Huainan, Emperor Xuānzong held a great feast to send him off and personally wrote a poem to him. In 858, when the Xuanshe Circuit officer Kang Quantai mutinied and expelled the governor Zheng Xun, who fled to Huainan's capital Yang Prefecture, Cui launched his troops to attack the Xuanshe rebels, so Emperor Xuānzong gave him the additional title of governor of Xuanshe. Later in the year, he captured Xuanshe's capital Xuan Prefecture, killing Kang and his coconspirators. He then resigned the additional post of governor of Xuanshe, which was then given to Wen Zhang.
During Emperor Yizong's reign
Early in the Xiantong era of Emperor Xuānzong's son and successor Emperor Yizong, Cui Xuan was transferred to Shannan East Circuit, and later to Jingnan Circuit. In 868, with soldiers originally from Xusi Circuit having launched a mutiny under the leadership of Pang Xun at their defensive post at Gui Prefecture as they wanted to return home to Xusi, Cui launched his troops and prepared for a confrontation with them; as a result, the Xusi mutineers did not dare to enter Jingnan, and instead crossed into Jiangxi Circuit and Huainan to head for Xusi. It was said that the imperial government much approved of Cui's preparedness. Cui eventually died at his post at Jingnan, but the year is not known. His son Cui Hang would serve as a chancellor during the reign of Emperor Yizong's son Emperor Xizong.
