He hailed from Lu 魯 (modern Qufu 曲阜, Shandong) and was a 14th-generation descendant of Confucius. In the age of 20 he was made Court gentleman for consultation (yilang 議郞) and was soon appointed Grand Master of Remonstrance (jian dafu 諫大夫). Kong Guang was charged with non-compliance and dismissed.
During the reign of Emperor Cheng 漢成帝 (r. 33-7 BCE) he was appointed erudite (boshi 博士) at the National University (taixue 太學). He regularly travelled through the empire and reported misgovernment and the suffering of the people to the throne. The emperor therefore promoted him to a position in the Imperial Secretariat (shangshusheng 尚書省), where Kong Guang was used for his expertise in administrative and legal matters and soon made Director of the Secretariat (shangshu ling 尚書令).
He later served as Grand Master of Splendid Happiness (guanglu dafu 光祿大夫), then Chamberlain for attendants (guanglu xun 光祿勳). Kong Guang was a honest, upright and diligent state official who clearly divided between official and private affairs. In 15 BCE he was made Censor-in-chief(a> (yushi dafu 御史大夫), in 7 BCE Counsellor-in-chief (chengxiang 丞相) and was concurrently given the title of Marquis of Boshan 博山侯.
Two years later he was dismissed because he had differences with Empress Dowager Fu 傅太后. When the Empress Dowager died in 2 BCE he returned to the office of Counsellor.
Emperor Ping 漢平帝 (r. 1 BCE-5 CE) made Kong Guang Grand Mentor of the Heir Apparent (taizi taifu 太子太傅) and Grand Minister of Education (dasitu 大司徒).
At that time Wang Mang 王莽 was already the strong man in the empire, a circumstance that Kong Guang was deeply worried about, so that he asked to be allowed to retire. He returned home and studied Xiahou Sheng's 夏侯勝 version of the Confucian Classic Shangshu 尚書 "Book of Documents" and instructed the disciples flocking to his door.
Kong Guang's posthumous title is Marquis Jianlie 簡烈侯.