Li Changchun was born in February 1944 in modern-day Dalian, Liaoning, then administered by the Empire of Japan as "Dairen", Kwantung Leased Territory. He joined the Communist Party of China in 1965 and graduated with a degree in electrical engineering from the Harbin Institute of Technology in 1966. In 1983, at age 39, he became the youngest mayor and Party secretary of a major city, of Shen Yang, the capital of Liaoning. In 1982, he was also made an alternate member of the Central Committee of the Communist Party of China at the age of 38, the youngest member of the body at the time. In 1987, he became governor of Liaoning province, a post he kept until 1990. As governor, mainland China's first expressway was built in the province, linking the cities of Shen Yang and Dalian. In addition, Li pushed for the reform of state-owned enterprises, aiming to decrease state involvement in their operations.

After General Secretary Zhao Ziyang was purged from the party leadership in 1989 during the fallout from the Tiananmen Square protests that same year, Li was initially also thought to have been removed from the leadership because he was a supporter of Zhao. Li's appearance on state television weeks later showed that this was not the case.

In 1990, Li was transferred from his job in Liaoning province to central Henan province. In his memoirs, Li recounted that he was ill-prepared for his new assignment and felt homesick. The central authorities had not given him much prior notice about his transfer, and did not inform him why he was being moved or facilitate an orderly transition process. Li, as a result, was somewhat critical of the party's transfer process but nonetheless duly accepted his new assignment. He had succeeded then-Henan governor Cheng Weigao, who had been transferred to Hebei province as part of a three-province 'leader swap' orchestrated by the party's Leaders. Henan, a populous agricultural province without a strong industrial base, presented Li with significant challenges, and Li had experienced unease settling into his new home.

Two years later, in 1992, Li was promoted to party chief of Henan. It would be Li's first job as "first-in-charge" of a province. Being accustomed to serving in government administration, Li's tenure in Henan was his first taste of being in charge of party affairs. Li said that initially being the top leader in the province made him uncomfortable as he had to shoulder all responsibility, especially at a time when other regions were developing economically at a pace much faster than that of Henan. Overall, his tenure in Henan was seen as mediocre. Rural incomes remained stagnant during his term, and his government was also blamed in a scandal involving tainted blood which led to the spread of AIDS in the province.

Li was promoted to the Politburo of the Communist Party of China in 1997, largely due to having secured the patronage of the party leader and President Jiang Zemin. In his memoirs, Li said that he was surprised at having been appointed to the Politburo. In 1998, Jiang dispatched Li to serve as Guangdong Party Secretary. It was said that Jiang wanted to use Li as a counterbalance to the entrenched local political establishment composed mostly of people native to the province. In Guangdong, Li cracked down on corruption to "put the house in order." In the wake of the Asian Financial Crisis of 1997-98, Li set up a special task force to evaluate what to do with non-performing loans owed by two of the province's largest financial companies. He appointed former central bank deputy governor Wang Qishan to oversee the task force. Li shook up the local banking sector and closed a plethora of local credit unions and agencies. He also increased access to the legal aid system for the poor in the province. His tenure in Guangdong was seen as largely successful, having averted the brunt of the Asian Financial Crisis and also bringing Guangdong back to the political control of the central leadership under Jiang Zemin.

In his position as China's propaganda chief from 2002 to 2012, Li was said to have contributed heavily to China's censorship campaign and frequently ordered media to downplay or not report on certain events. In 2006, he told the members of the All-China Journalists Association to "closely encircle the overall work of the party and state". Li approved the construction of the National Museum in 2006 after a series of disputes and delays about the building of the museum. He was the guest of honor at the opening of the National Center for the Performing Arts.

Li's tenure in Guangdong made him one of Jiang's favourites and as such Jiang was preparing to groom him for succession for the premiership upon incumbent Premier Zhu Rongji's scheduled retirement in 2003. However, Zhu had been favouring Wen Jiabao for the Premier office, and criticized Li over his handling of an "export rebate fraud" scandal in the coastal city of Shantou in 2000, which took place during Li's term as Guangdong party chief. Li's intention to promote Huang Liman, a female friend of Jiang's who was considered incompetent, to the party chief position in the coastal city of Shenzhen became a sticking point for Jiang's political opponents.

Li has put his support behind a number of creative projects that might otherwise have been censored by the government. He supported Zen Shaolin, a music, dance and martial arts show intended to increase tourism that opened in 2007 in Henan, despite the producers' concerns that a celebration of religion and sacred music would be opposed by the government. Li allowed a 2009 movie Nanking! Nanking! by Lu Chuan to continue running in theaters in the face of strong pressure from nationalists who objected to the sympathetic characterization in the film of a Japanese soldier. The film was one of ten chosen to help commemorate 60th anniversary of the People's Republic of China.