His older brother Dou Deming (竇德明) served on the staff of Emperor Taizong of Tang while he was the Prince of Qin.
Dou Dexuan himself was said to start his career in civil service when he was first an imperial university student during the reign of Emperor Yang of Sui.
In 664, he was also made acting Zuo Xiang (左相) -- the head of the examination bureau of government and a post considered one for a chancellor.
In 665, as Emperor Gaozong was planning to sacrifice to heaven and earth at Mount Tai, and he had Dou and Li Ji serve as his attendants.
The people who heard this exchange admired Dou for his honesty—particularly Li Ji, who commented, "It is a good thing that Xu Jingzong is knowledgeable.