He was praised by historians for his insistence on being morally upright, and for being a just administrator of the law during his time as Xuanzong's senior chancellor.
Soon thereafter, he was made Huangmen Shilang (黃門侍郎), the deputy head of the examination bureau of government (門下省, Menxia Sheng).
Meanwhile, other officials, including Su Xiang (蘇珦), Xu Jian (徐堅), and Yin Sizhen (尹思貞), pointed out that it was inappropriate to execute anyone during the summer.
Soon thereafter, Wu Sansi had Song sent out of the capital to serve as acting prefect of Bei Prefecture (貝州, in modern Xingtai).
Emperor Zhongzong died suddenly in 710—a death that traditional historians believed to be a poisoning carried out by Empress Wei and her daughter Li Guo'er the Princess Anle, so that Empress Wei could become "emperor" like Wu Zetian and Li Guo'er could be crown princess.
Song Jing was made the secretary general of Luo Prefecture (洛州), which contained the eastern capital Luoyang.
Emperor Ruizong made Song the minister of civil service affairs (吏部尚書) and gave him the designation Tong Zhongshu Menxia Sanpin (同中書門下三品), making him a chancellor de facto.
During Emperor Zhongzong's reign, the civil service affairs system was subject to great abuses by powerful individuals at court, particularly while the chancellors Cui Shi and Zheng Yin were in charge — as they were allowing bribes and pleas by powerful individuals to influence them, going as far as naming reserve officials two years in advance.
Emperor Ruizong initially agreed (with the exception that he believed that Luoyang was too far, and therefore was ready to send Princess Taiping to Wu Youji to Pu Prefecture (蒲州, roughly modern Yuncheng, Shanxi) instead), but after Princess Taiping found out and objected vehemently, Li Longji, fearful of consequences, disavowed and denounced Song and Yao.
As of 715, he was serving as chief imperial censor (御史大夫, Yushi Daifu), when, on one occasion, he was accused of not ordering for greater force to be applied when a criminal was being caned, and he was demoted to be the prefect of Mu Prefecture (睦州, in modern Hangzhou).
Meanwhile, Yao was drawing displeasure from Emperor Xuanzong due to accusations that his sons and associate Zhao Hui (趙誨) were corrupt.
Emperor Xuanzong, who was then preparing to go to Luoyang, thus recalled Song to serve as the minister of justice (刑部尚書, Xingbu Shangshu) and be in charge of Chang'an.
He sent one of his close eunuch associates, Yang Sixu (楊思勗), to Guang Prefecture to escort Song back to the capital.
Around the new year 717, he removed Yao and fellow chancellor Yuan Qianyao from their chancellor posts and replaced them with Song and Su Ting—naming Song Huangmen Jian (黃門監, the head of the examination bureau, which by that point had been renamed Huangmen Sheng (黃門省)) and acting minister of civil service affairs.
With Yao having distinguished himself as one who modified the regulations to fit the situation and Song known for following the laws strictly and fairly, it was said that while their styles were different, they were both considered great chancellors of Emperor Xuanzong's reign.
One day, while Emperor Xuanzong was watching a play, an actor dressed as the Hanba, and stated: There are over 300 people saddled with unfair judgments.
Song sent the imperial censor Xiao Yinzhi (蕭隱之) to the region to be in charge of collecting the damaged coins and taking them out of circulation.
Song quickly determined who were the leaders and had them executed, and at his request, Emperor Xuanzong pardoned the rest of the detainees.
In 729, Song was made You Chengxiang (右丞相), one of the heads of the executive bureau of government (尚書省, Shangshu Sheng).
Song argued fervently against it, believing that this would cause capable people to be stuck in low positions, but Emperor Xuanzong did not agree.