It is not known when Cui Dan was born. He was from the "Lesser Branch" of the prominent Cui clan of Qinghe, and his ancestors originally claimed ancestry from the ruling house of the Spring and Autumn period state Qi. Cui Dan's traceable ancestry included officials of Han Dynasty, Liu Song, Northern Wei, and Tang Dynasty. His grandfather Cui Ji served as a staff member for a crown prince, while his father Cui Chui served as a deputy chief imperial censor. Cui Dan had at least seven brothers — older brothers Cui Bin, Cui Feng, Cui Yan, Cui Xun, Cui Han, and Cui Shan, and younger brother Cui Fu. Among the brothers, Cui Bin was the most well-known and one whose acts were most well-recorded in the official historiesOld Book of Tang and New Book of Tang. Six of the brothers would eventually reach high level positions within the Tang government.
Career before chancellorship
Cui Dan's biographies in the Old Book of Tang and the New Book of Tang had different descriptions of his career path, both agreed that he passed the imperial examinations in the Jinshi class. The Old Book of Tang mentioned that he served in an unspecified office before serving as an imperial censor with the title Jiancha Yushi and then as Kaogong Langzhong, a supervisory official at the ministry of civil service affairs. According to the Old Book of Tang, then, in 829, during the reign of Emperor Wenzong, while remaining as Kaogong Langzhong, he was also made an imperial scholar, and thereafter was made Zhongshu Sheren, a mid-level official at the legislative bureau of government. As of 832, he was no longer imperial scholar. The New Book of Tang, while discussing the same period of his career, mentioned that after he passed the imperial examinations, he served as the sheriff of Weinan County, and after some promotions became Xingbu Langzhong, a supervisory official at the ministry of justice. He subsequently served as the deputy military governor under the former chancellor Du Yuanying, the military governor of Xichuan Circuit. After that, Cui was recalled to the capital Chang'an to serve as the deputy minister of public works, and an imperial scholar at Jixian Hall — an event that the Old Book of Tang also described and placed in 834, while further mentioning at that time, he was also made acting minister of rites. The two biographies' description of Cui's career thereafter did not significantly diverge. According to the Old Book of Tang, he thereafter served as deputy minister of defense, and also was put in charge of selecting officials for the eastern capital Luoyang. When Emperor Wenzong once summoned the officials in selecting of commissioning officials to discuss with them the criteria for selecting officials. During that conversation, he asked Cui what Cui would do with candidates who were not capable; Cui responded that he would send them to the border regions, a response that Emperor Wenzong did not agree with, pointing out that that meant that the people living on the borders would be mistreated. Nevertheless, he subsequently made Cui the deputy minister of civil service affairs. In 837, Cui was sent out of Chang'an to serve as the governor of Xuanshe Circuit, as well as the prefect of its capital Xuan Prefecture. In 839, he was recalled to Chang'an to serve as the minister of worship.
Chancellorship
In fall 839, Cui Dan, while still serving as the minister of worship, was made a chancellor de facto with the designation Tong Zhongshu Menxia Pingzhangshi. He thereafter was also made Zhongshu Shilang, the deputy head of the legislative bureau, and given the honorary title Yinqing Guanglu Daifu. After Emperor Wenzong died in 840 and was succeeded by his brother Emperor Wuzong, although Cui's colleagues Yang Sifu and Li Jue were removed from their posts, and Li Deyu became the leading chancellor, Cui remained chancellor. In 841, when Emperor Wuzong came to believe in allegations that Yang and Li Jue had supported other candidates to succeed Emperor Wenzong rather than him and wanted to put Yang and Li Jue to death, Li Deyu, at the advice of Du Cong, interceded, along with Cui and their other chancellor colleagues Chen Yixing and Cui Gong, and Emperor Wuzong spared Yang and Li Jue's lives, although exiling them far away from the capital.
Career after chancellorship
Late in 841, Cui Dan was sent out of Chang'an to serve as the military governor of Xichuan Circuit. Early in the subsequent reign of Emperor Wuzong's uncle Emperor Xuānzong, Cui was made the military governor of Huainan Circuit, carrying the title of Tong Zhongshu Menxia Pingzhangshi as an honorary title. He died while still serving at Huainan, but the time is not known.
