Kang Sheng’s early life is documented as if relating to different people. He was born in Shandong to a gentry family and named Zhao Rong in 1903, or as Zhang Zongke, perhaps in 1893, 1898, or 1899. Kang studied at Shanghai University and joined the party, both in 1924-25. In 1920, Kang took preparatory courses at Qinghua University, then taught in a rural school in Zhujiang, Shandong before resuming studies in Shanghai. He joined the CCP in 1924 or 1925 in Shanghai, where he worked as a labor organizer under different alias such as Zhang Shaoqing and Zhao Rong, and took part in the unsuccessful Communist uprising in 1927. As a Shanghai district CCP leader, he participated in uprisings in that city under the leadership of Zhou Enlai ; when the uprising was put down by Kuomintang, Kang escaped from Shanghai. He was briefly a CCP department chief in the Jiangsu Provincial Committee in 1928 and then joined the surviving Communist cadres in the rural areas, and in 1930.
One report has Kang as CCP Central Committee Organization Department Director , politburo member and Central Committee secretary before being sent as a permanent member of the CCP delegation to the Comintern Executive in Moscow. Other reports say that he studied in Moscow from as early as 1930, and remained there until 1937, working in the Comintern under Wang Ming and, at least at times, along side Chen Yun. All three returned to China, to Yenan, in 1937 and taught at the Anti-Japanese University .
Kang arrived in Mao Zedong's base at Yan'an sometime in the late 1930s, with the latest inside information on Moscow’s thinking, and was appointed to the CCP CC Secretariat in 1938. He may have already realized that Wang Ming was falling out of favor, and Zhang Guotao was originally selected by the Comintern to replace Wang. Kang Sheng allied himself with Mao to destroy Wang's faction within the party, seeing Wang as the greatest enemy at the time.
At Yan'an, Kang became a close friend of Jiang Qing, who may have been Kang's maid during his youth in Shandong, and who became a second-rate young actress in Shanghai and a newly converted Communist. He introduced her to Mao Zedong, who later married her.
In June 1942, Kang was said to have been spending all his time with Mao. There are conflicting reports about his role, or fate during the 1942 Rectification Campaign : One source says he was criticized, and then replaced Li Weihan as head of the CCP Party School, while another says he was responsible for turning Mao’s innocent effort to educate newly arrived cadres into a violent purge. In his August 1943 speech, Kang explained how he and his colleagues used rectification to expose spies and trick anti-Party elements into reveling themselves. The strategy calls to mind the 1950s Hundred Flowers Movement and its aftermath.
During the 1946-49 Chinese Civil War, Kang was named to CCP chief of Shandong Province and second Deputy General Secretary of the party’s East China Bureau.
Kang played no visible public role in the early years of the PRC: it is said that the enmity of President Liu Shaoqi and Premier Zhou Enlai kept his role to a minimum. He resurfaced in the mid- 1950s following his active role in the purge of military leader Peng Dehuai, and apparently resumed control of the CPC security apparatus. He became Mao's personal agent in the intra-Party struggles that began with the "Anti-Rightist Campaign" of 1959 and culminated in the 1966-76 Cultural Revolution. As a close associate of Jiang Qing, he became a member of the Party Secretariat under Deng Xiaoping in 1962. By 1966, he became an "adviser" to the Cultural Revolution Group under the Central Committee, and a member of the Politburo's Standing Committee. His actions set into motion the Cultural Revolution, which he created in order to increase his personal power and rank within the CPC.
Kang was closely involved in the Cultural Revolution purges which resulted in the downfall of Peng Dehuai, Liu Shaoqi, Deng Xiaoping, Marshal Lin Biao, Marshal He Long, and many other leading CPC figures. His position in the CPC rose with the fall of each of these high-ranking leaders. Kang's campaigns of state terror reached as far as Inner Mongolia, where he instigated a deadly witchhunt for members of the defunct Inner Mongolian People's Party, which had once existed as a separatist party but was disbanded and absorbed by the CPC long before; and Yunnan Province, where thousands were died. In this wave of persecution, Kang Sheng adopted a different tactics than that of Yan'an: learning from his dismay from Zheng Feng movement more than two decades earlier, Kang Sheng cleverly stayed in the background this time, and encouraged the Red Guards and the general populace to eliminate the class enemy, as well as fighting each other, hence shifted the responsibility ostensively away from himself and Mao. As a result, Mao was pleasd with Kang Sheng for achieving the elimination of the so-called class enemy while shifting the responsibility to others , and Kang Sheng's position was further strengthened in the Cultural Revolution.
Kang also left a lasting imprint on China's foreign policy. While the mainstream of the CPC leadership cultivated Prince Norodom Sihanouk as Cambodia's anti-Western and anti-imperialist leader, Kang advocated that Khmer Rouge leader Pol Pot was the real revolutionary leader in the Southeast Asian nation. As a result, Pol became the recipient of Chinese aid for years to come.
At the apex of his power, Kang ranked fourth behind Mao, Lin Biao, and Zhou Enlai. His last service to Mao was the 1976 campaign to criticise "rightist deviationism," which was aimed at Zhou Enlai and Deng Xiaoping, though Kang died of cancer in late 1975 before it was launched. Even before drawing his last breath, Kang had called Mao's interpreters and proteges Nancy Tang and Wang Hairong to his hospital, accusing Jiang Qing of having betrayed the CPC to the KMT before the Communist victory. Kang may have forecasted Jiang's fall, or he may merely have been speculating as to her fate.
Had Kang not died, he would certainly have been removed from power along with the after Mao's death. In a secret speech delivered in 1978, Hu Yaobang compared Kang to secret police chiefs Felix Dzerzhinsky and Lavrenty Beria. He was posthumously expelled from the Party in 1980, and his remains were removed from Babaoshan Revolutionary Cemetery in Beijing, where the remains of many prominent CPC leaders are interred.
In contrast to Dzerzhinsky, who was a pious believer in communism and who lived a very simple and modest daily life, Kang lived an extravagant and corrupt lifestyle. As the best calligraphist among senior leaders of CPC, as well as a painter, art and antique connoisseur, poet, and historian, Kang had a great appetite for valuable antiques and used his power to embezzle many from the Forbidden City and from the storehouses of the Cultural Relics Bureau during the Cultural Revolution, a fact uncovered only after his death. According to the audits by the Chinese government and researches by the Japanese, Kang Sheng was both the very first millionaire and the first multi-millionaire in China, based on the value of artifacts he owned in 1970s price. Furthermore, it was rumored that he had kept an affair with the sister of his wife Cao Yi'ou for quite a long time, and he built several villas for their rendezvous.