As of 658, during the reign of Emperor Gaozong, Xin Maojiang was serving as the chief judge of the supreme court, when he was made Palace Attendant, the head of the Chancellery and a default chancellor. In summer 659, he and fellow chancellor Xu Jingzong were put in charge of an investigation of an alleged treason plot by the low level officials Wei Jifang (韋季方) and Li Chao (李巢), although it was Xu, an ally of Emperor Gaozong's powerful wife Empress Wu (later known as Wu Zetian), who spearheaded the investigation who ultimately convinced Emperor Gaozong that his uncle and chancellor Zhangsun Wuji, who had implicitly disapproved of Empress Wu's ascension (replacing Emperor Gaozong's first wife Empress Wang) in 655 by refusing to concur in the decision, was part of the plot. As a result of further investigations ordered by Emperor Gaozong (which not only Xin and Xu, but also Li Ji, Ren Yaxiang, and Lu Chengqing were involved in), Zhangsun was first exiled and then forced to commit suicide. Xin himself died around new year 660, and if he was given a posthumous name or noble title, neither is recorded in history.