In 705, during the second reign of Emperor Zhongzong, Xiao Song was made a military officer at Ming Prefecture (洺州, in modern Handan, Hebei).
Early in the Kaiyuan era (713–741) of Emperor Ruizong's son Emperor Xuanzong, Xiao Song became Zhongshu Sheren (中書舍人), a mid-level official at the legislative bureau, serving alongside Cui Lin (崔琳), Wang Qiu (王丘), and Qi Huan (齊澣).
He participated in government reactions to a Yellow River levee break at Bo Prefecture (博州, roughly modern Liaocheng, Shandong).
In 726, Xiao was given the title of minister of defense (兵部尚書, Bingbu Shangshu) and made the military governor (jiedushi) of Shuofang Circuit (朔方, headquartered in modern Yinchuan, Ningxia).
In 727, having been aggravated by constant attacks by the Tang general Wang Junchuo (王君㚟) the military governor of Hexi Circuit (河西, headquartered in modern Wuwei, Gansu), generals We Tadra Khonglo ("Xinuoluo Gonglu" (悉諾邏恭祿) in Chinese) and Zhulongmangbu (燭龍莽布) of the Tibetan Empire launched a major attack on Gua Prefecture (瓜州, roughly Jiuquan, Gansu) and captured its prefect Tian Yuanxian (田元獻) and Wang Junchuo's father Wang Shou (王壽).
In the aftermaths, Wang Junchuo falsely accused the tribal chieftains of the Uyghur Khaganate, Huns (渾), Kibirs (Ch.
Xiao retained a number of officials that were considered capable—Pei Kuan (裴寬), Guo Xuji (郭虛己), and Niu Xianke, and recommended the general Zhang Shougui (張守珪) to serve as the prefect of Gua Prefecture.
Emperor Xuanzong, hearing this, gave him the honorific title Yinqing Guanglu Daifu (銀青光祿大夫).
When the unsealed reports of the victory arrived at the capital Chang'an, Emperor Xuanzong was pleased, and he recalled Xiao to Chang'an to serve as minister of defense; he also gave Xiao the designation Tong Zhongshu Menxia Pingzhangshi (同中書門下平章事), making him a chancellor de facto.
Xiao wanted to recommend his friend and old colleague Wang Qiu, who was then serving as a senior advisor at the examination bureau (門下省, Menxia Sheng).
Xiao responded: I have received great grace to serve as chancellor, and I have reached the peak of honor and wealth.
If you are tired of me, I will not even be able to keep my head.Xiao then began to cry, and Emperor Xuanzong was touched, responding, "Leave my presence for the time being.
In 734, when a major earthquake occurred at Qin Prefecture (秦州, roughly modern Tianshui, Gansu), causing more than 4,000 deaths, Emperor Xuanzong had Xiao head the disaster relief efforts.
Xiao had long been taking herbal medicines, and once he left the chancellorship, he began to grow his own herbs for his own use.