As of the reign of Emperor Gaozong's wife Wu Zetian (r. 690–705), Wang Jun served as Dianzhong Shi Yushi (殿中侍御史), a low-level imperial censor and also carried the honorific title of Chaosan Daifu (朝散大夫).
Near the end of the Jinglong era (707-710) of Wu Zetian's son and successor Emperor Zhongzong, Wang Jun became the commandant at Gui Prefecture (桂州, roughly modern Guilin, Guangxi).
All of this made things very difficult for Emperor Xuanzong, forcing him to stay a little further back to avoid giving excuses to his aunt.
Sometime thereafter, he was made the deputy minister of husbandry (太僕少卿, Taipu Shaoqing), responsible for the herds of the Longyou (隴右, eastern Gansu) region.
He was also made the commandant at Yuan Prefecture (原州, roughly modern Guyuan, Ningxia), and his son Wang Ting (王珽) was given the honorific title of Chaosan Daifu.
Wang, believing that the Göktürks who surrendered would try to flee back to the Göktürk state, suggested that they be forcibly moved into the heart of the empire to prevent them from doing so.
In 721, the non-Han Chinese in the region rebelled under the leadership of Kang Daibin (康待賓) and attacked Xia Prefecture (夏州, roughly modern Yulin, Shaanxi).
Emperor Xuanzong ordered Wang and Guo Zhiyun (郭知運), the military governor (jiedushi) of Longyou Circuit to attack Kang.
Emperor Xuanzong blamed this on Wang and demoted him to be the prefect of Zi Prefecture (梓州, roughly modern Mianyang, Sichuan).
In 722, Emperor Xuanzong recalled Wang to serve on as the head of the household for his crown prince Li Siqian, and created him the Duke of Zhongshan.
Subsequently, when Emperor Xuanzong went on a tour of the northern regions, he made Wang Jun the mayor of Taiyuan Municipality (converted from Bing Prefecture).
In summer 723, Wang was made the minister of defense (兵部尚書, Bingbu Shangshu) and given the designation Tong Zhongshu Menxia Sanpin (同中書門下三品), making him a chancellor de facto.
In winter of that year, Emperor Xuanzong was set to offer sacrifices to heaven and earth, and he ordered Wang to return to the capital to attend.
Still, Wang Jun was demoted to be the prefect of Qi Prefecture (蘄州, roughly modern Huanggang, Hubei) on the charge of disobeying the imperial edict.