He was traditionally considered a skilled politician who maintained his high position throughout his lengthy career, but not a capable chancellor.
[4] Because Du Cong's heritage, he entered civil service early, and as his third assignment he served as a staff member of the Crown Prince.
In 834, he was recalled to government service as the military governor of Zhongwu Circuit (忠武, headquartered in Xuchang, Henan).
[9] Around the new year 838,[10] Du was recalled to Chang'an to serve as the minister of public works (工部尚書, Gongbu Shangshu) and acting director of finances.
At that time, Princess Qiyang died; as a result of observing a mourning period for her—as it was customary for princesses' husbands to observe a three-year mourning period for them, although that was not required of ordinary widowers—he did not meet Emperor Wenzong to thank him for the commission, which surprised Emperor Wenzong.
In 838, Du was made minister of census (戶部尚書, Hubu Shanshu) and continued to act as the director of finances.
In 841, after further accusations by Qiu against Yang, Li Jue, as well as two eunuchs that Emperor Wenzong had favored, Liu Hongyi (劉弘逸) and Xue Jileng (薛季稜), Emperor Wuzong ordered Liu and Xue to commit suicide, and sent messengers to Tang Prefecture (潭州, in modern Changsha, Hunan), where Yang was serving as the governor of Hunan Circuit (湖南), and Gui Prefecture (桂州, in modern Guilin, Guangxi), where Li Jue was serving as the governor of Gui District (桂管), to order Yang and Li Jue to commit suicide as well.
[11] As of 844, Du was serving as the military governor of Huainan Circuit (淮南, headquartered in modern Yangzhou, Jiangsu), when Emperor Wuzong issued an order to the eunuch monitor of Huainan Circuit that he should select 17 prostitutes who were capable in drinking games and send them to the palace.
The eunuch monitor asked Du to be involved in the selection process, and further contemplated training regular women to learn the drinking games and then submitting them.
Du, pointing out that at that time the imperial treasury was exhausted, argued for Guo to be tolerated, thus drawing Emperor Wuzong's displeasure.
When Emperor Xuānzong received report of this, he sent the chancellor Cui Xuan to Huainan to serve as its military governor, and made Du a senior advisor to the Crown Prince, but with his office at the eastern capital Luoyang.
[2] As of 861, by which time Emperor Xuānzong had died and been succeeded by his son Emperor Yizong, Du Cong was back at Chang'an and serving as Zuo Pushe (左僕射, one of the heads of the executive bureau of government (尚書省, Shangshu Sheng)) and the director of finances, when he was made Menxia Shilang (門下侍郎), the deputy head of the examination bureau (門下省, Menxia Sheng) and chancellor again with the designation Tong Zhongshu Menxia Pingzhangshi.
Emperor Yizong, in response, issued an edict ordering that Qin be executed and that his assets and family be forfeited.