He hailed from Yanling 鄢陵 (modern Yanling, Hebei) and served in his home commandery as a small employee in the local administration because he professed in the Confucian Classics. Censor-in-chief (yushi dafu 御史大夫) Zhang Zhong 張忠 made him a scribe in his retinue and recommended him for the position of court gentleman for consultation (yilang 議郎). Sun Bao was soon appointed Grand master of remonstrance (jian dafu 諫大夫) and then regional inspector (cishi 刺史) of Yizhou 益州.
Because he had offended Minister of War (dasima 大司馬) Wang Yin 王音 he was put into jail, but so many people submitted pleas to pardon him so that he was released and transferred to the post of regional inspector of Jizhou 冀州. Later on he was made governor (taishou 太守) of the commandery of Guanghan 廣漢, where he was able to bring the unruly native population into submission. Sun Bao was therefore promoted to metropolitan magistrate (jingzhao yin 京兆尹) and fell again into disgrace because he was accused by the powerful minister Chunyu Zhang 淳于長.
Emperor Ai 漢哀帝 (r. 7-1 BCE) again appointed him Grand master of remonstrance and then to metropolitan commandant (sili xiaowei 司隸校尉). Vice Director of the Imperial Secretariat (shangshu puye 尚書僕射) Zheng Chong 鄭崇 charged him with treason, so that Sun Bao was demoted to commoner, a position that did not allow him to occupy and government post.
Emperor Ping 漢平帝 (r. 1 BCE-5 CE) promoted him again, this time to Grand Master of Splendid Happiness (guanglu dafu 光祿大夫), and then to Chamberlain for the national treasury (dasinong 大司農). A third time he was accused, this time by Chen Chong 陳崇 because of unfilial behaviour. Sun was dismissed and later died as a private person.