Schooled under Cai E, Li graduated from the Guilin Military Cadre Training School and became a platoon commander in the formidable force of Lu Rongting's subordinate Lin Hu in 1916, then served in the northern expedition during 1918 in Hunan, his bravery earned him a promotion to battalion commander.
Li Zongren accompanied Lin Hu and Lu Rongting into Guangdong and led the rear guard when the Old Guangxi Clique forces retreated before Chen Jiongming's attack. Most of Lin Hu's officers were former bandits and militia recruited earlier by Lin from the Zhuang areas of Guangdong. They defected to the Guangdong forces, taking their units with them. Li Zongren his battalion, shrunk to about one thousand men "sank into the grasses." Li, intending to become more than a bandit, began building a personal military machine of professional units of soldiers that were the equal of any number of bandits or Zhuang irregulars that Lu Rongting drew on in his war to re-establish his power in Guangxi.
Li joined the Kuomintang in 1923, when he already controlled a considerable numbers of troops in northern Guangxi. Having wiped out the bandits, local warlords, and remnant forces of the north, he joined Huang Shaohong and Bai Chongxi in the spring of 1924 to form the new Guangxi Clique and create the Guangxi Pacification Army. Li Zongren was the Commander in Chief, Huang Shaohong the deputy Commander, and Bai Chongxi the Chief-of-Staff. By August they had defeated Lu Rongting and driven other contenders out of the province. Li Zongren was military governor of Guangxi from 1924-25, and from 1925 to 1949, Guangxi remained under Li Zongren's influence.
Li went on to be the commanding general of the Seventh Army in the Northern Expedition and captured Wuhan in 1927. Appointed commander of the 4th Army Group, composed of the Guangxi Army and other provincial forces amounting to 16 corps and six independent divisions. In April, 1928, Li Zongren, with Bai Chongxi led the Fourth Army group to advance on Beijing, capturing Handan, Baoding, and Shijiazhuang, by June 1. Zhang Zuolin withdrew from Beijing on June 3, and Li's army seized Beijing and Tianjin.
At the end of the Northern Expedition, Chiang Kai-shek began to agitate to reorganize the army in a military conference in 1929, the fact that it would alter the existing territorial influences among the cliques in the party quickly aggravated the relationships between the central government and the regional powers. Li Zongren, and the New Guangxi clique were the first to break off relations with Chiang in March 1929. This effectively started the confrontation which lead to the Central Plains War. After the Guangxi Army captured Yueyang, Chiang's forces cut them off from behind. The Guangxi Army was eventually forced to withdraw back to Guangxi.
Following defeat in that civil war, Guangxi allied with Chen Jitang after he became chairman of the government of Guangdong in 1931, and turned against Chiang Kai-shek. Another civil war would have broken out if there had been no September 18 Incident, which prompted all sides to unite against the Empire of Japan.
During the Second Sino-Japanese War (1937-1945), Li participated in several battles including the Battle of Tai'erzhuang, Battle of Xuzhou, Battle of Wuhan, Battle of Suixian-Zaoyang, 1939-40 Winter Offensive, Battle of Zaoyang-Yichang, Central Hopei Operation, and Battle of South Henan. From 1943 to 1945 Li was made Director of the Generalissimo's Headquarters.
According to Jonathan D. Spence, Li was one of Chiang Kai-shek's best generals and "fought a brilliant battle (at Xuzhou), luring the Imperial Japanese Army into an trap and killing as many as 30,000 of its combat troops" in 1938.
After the war, Li was given the post of Director of the Peiping Field Headquarters from 1945 to 1947. This was a post without effective power, he was sidelined from command in the early part of the Chinese Civil War.
On 28 April 1948, Li was elected by the National Assembly as the vice-president, five days after his political opponent, Chiang Kai-shek became the president. (Chiang had supported Sun Fo's candidacy instead.) The day after Chiang resigned on 21 January, 1949 as a response to the Chinese Communist uprisings and several victories, Li became the acting president. Li attempted to negotiate with the communists in Beijing. Such "pacifist attacks" increased the already-strained Li-Chiang tension. Li's intended and never implemented Seven Great Peace Policies were:
Li's attempts to carry out these policies, faced varying degrees of opposition from Chiang's supporters.
When the capital Nanjing fell to Communist forces in April 1949, Li led the evacuation of the government to Guangzhou. Li hoped to launch a counter-attack against the Communists from Guangdong, much like the KMT advance against the warlords during the Northern Expedition. Chiang Kai-shek supported retreating to the interior to regroup, as he had done during the Second Sino-Japanese War. So in November 1949, when Guangzhou fell to the Communists, Chiang relocated the government to Chongqing while Li effectively surrendered his powers and flew to New York for treatment of his chronic duodenum illness at the Hospital of Columbia University. Li visited the United States President Harry S. Truman and denounced Chiang as a "dictator" and "usurper." Li doughtily vowed he would "return to crush" Chiang's movements once he went back to China.
In December 1949, Chongqing fell too, and Chiang relocated his government to Taipei, but he did not formally reassume the presidency until March 1, 1950. In January 1952, Chiang commanded the Control Yuan now in Taiwan to impeach Li in the "Case of Li Zongren's Failure to carry out Duties due to Illegal Conduct" (李宗仁違法失職案), and officially relieved Li of the position as vice-president in the National Assembly in March 1954. Li became a communist sympathizer and moved to Beijing with the support of Zhou Enlai on July 20, 1965. He died of duodenum cancer in Beijing at 78.
Li's residence in mainland China is viewed by some Chinese communists as a defect that caused Li to "patriotically return to the embrace of his Motherland with smiles" -- something similarly in perception to the former Qing Emperor Puyi's "reformation". To this day, some Nationalists supporters view him as a traitor to the democratic cause.
He co-wrote Memoirs of Li Zongren with historian Te-Kong Tang (唐德剛), which vehemently criticizes Chiang Kai-shek and analyzed Japan's strategic failure to conquer China. A more detailed and accurate account for Li's life is depicted in the less popular biography Wo De Gu Gong by his distant relative Namgo Chai.