He hailed from Xue 薛 (modern Tengxian 滕縣, Shandong) and was a jail overseer (yuli 獄吏) in his younger years. Inspite of his poor economic situation he began studying the Confucian Classic Chunqiu 春秋 "Spring and Autumn Annals" at the age of 40 and was selected as erudite (boshi 博士) for this classic during the reign of Emperor Wu 漢武帝 (r. 141-87 BCE).
He later retired from this post because he did not consent with the Emperor's politics against the steppe federation of the Xiongnu 匈奴. In 130 he was again appointed erudite and then transferred to the post of Left Chamberlain for the Capital (zuo neishi 左內史).
Emperor Wu highly esteemed him for his compliant character. In 126 he appointed him Censor-in-chief (yushi dafu 御史大夫), to years later to Counsellor-in-chief (chengxiang 丞相). Gongsun Hong was concurrently given the title of Marquis of Pingjin 平津侯. He was thus the first person of humble origins that had acheived such a high status. From then on it became a rule that the Counsellor-in-chief was automatically given the title of marquis.
Other court official did not like him much because he was a very upright and incorruptable person and esteemed inner values higher than pageantry and money. His retainers all came from his home town, and he did not allow excessive spending for clothes and food. When Liu An 劉安, the Prince of Huainan 淮南, rebelled, Gongsun Hong asked to be allowed to retire, yet Emperor Wu refused.
He died in office. His posthumous title is Marquis Xian 獻侯.
The imperial bibliography Yiwenzhi 藝文志 in the official dynastic history Hanshu 漢書 lists a book Gongsun Hong with a length of 10 chapters. It is lost.