Sun Li-jen was born in Jinnu, Lujiang, Chaohu, Anhui, with ancestry in Shucheng County. During the May Fourth Movement, he was part of the Scouts in the march at Tiananmen Square. In the same year (1919) he married Gong Xitao (龔夕濤) and was admitted in 1920 to Tsinghua University to study civil engineering. Sun played basketball at Tsinghua, becoming a star. He led the Chinese team to a gold medal at the 1921 Far Eastern Championship Games.

With a Boxer Rebellion Indemnity Scholarship, he transferred to Purdue University in the United States to complete his senior year in 1923, where he graduated in 1924. But in the United States, ideological zeal motivated him to dramatically change vocations and pursue a military career instead. China was in the middle of civil war, and Soviet and Japanese invaders seemed poised to devour China. Sun decided that he could better serve his divided nation as a soldier rather than an engineer.

He applied to the Virginia Military Institute, also in the United States, lying about his age by four years so that he would appear young enough to meet the school's admissions requirements. He graduated from VMI in 1927 and joined Generalissimo Chiang Kai-shek in the Northern Expedition against the warlord generals of the Beiyang Army. During the Second Sino - Japanese War and the latter war against the Chinese Communists in the Chinese Civil War, Sun Li-jen became a highly effective field officer and valued subordinate to Chiang Kai-shek.

Sun was a colonel and led his troops, a Tax Police Regiment, fighting Japanese during the Battle of Shanghai in 1937. He was badly wounded by rifle grenade fragments. After recovery from his injury, Sun led his troops back to the front line. Later, the Nationalist government formed National Salt Gabelle Brigade with 8 regiments. Four of the regiments later became New 38th Division with Sun as the Commanding General. His training center was located in Duyun in the province of Guizhou.

After two years training, Sun's New 38th Division was part of the forces Chiang Kai-shek sent into Burma to protect the Burma Road under General Du Yuming. Sun led Chinese forces to the relief of British forces trapped by the Japanese in the Battle of Yenangyaung. Although unable to stop the Japanese from cutting the Burma Road, Sun gained the respect of General William Slim, the Commander of the British 14th Army for his competence. Sun and his division retreated into India and became a part of 'X Force', the Chinese forces under the command of Joseph Stilwell, the American commander of all American and Chinese forces deployed in the "China Burma India Theater". Sun's division spearheaded Stilwell's 1943 drive to reconquer North Burma and re-establish the land route to China by the Ledo Road.

As the commander of the Army Training Command and deputy commander of the Republic of China Army in 1947, Sun moved one training facility to Taiwan, independent from the on-going civil war. Sun trained new officers and troops for the Nationalist government, hoping to change the tide of the civil war. The effort was too little too late in comparison with the massive numbers of troops defeated, but one of the divisions trained (201 Division of the 80th Army) was sent to Quemoy to help defend against the communist invasion in 1949. It was the front line defense force against the invasion of the communist troops. Later in 1950, Sun was named the Commander in Chief of the Republic of China Army, while also the commander of the Taiwan Defense Command, and the Army Training Command.

Because Sun was well respected by the Americans and rumors had it that the Americans would like to help him into power to replace Chiang Kai-Shek, Chiang and his son Chiang Ching-kuo were eager to remove him from power. Sun was assigned first as the ceremonial chief military adviser to President Chiang Kai-Shek in June 1954, preventing him from directly controlling any troops. Later in August 1955, he was accused and arrested, charged with attempting to stage a coup against Chiang Kai-shek. He remained under house arrest until exonerated March 20, 1988, shortly after Chiang Ching-kuo's death. He died in his Taichung home at the age of 89 (91 according to the Chinese calendar). His funeral was in full military honor with the presence of the Secretary of Defense. His reputation was cleared in 2001 after a government investigation into the alleged coup attempt. The Control Yuan formally cleared him of charges in January 2001.