Amin's rule was characterised by human rights abuses, political repression, ethnic persecution, extrajudicial killings, nepotism, corruption and gross economic mismanagement. The number of people killed as a result of his regime is estimated by international observers and human rights groups to range from 100,000 to 500,000.

Notable backers of Amin included Muammar al-Gaddafi's Libya, the Soviet Union and East Germany, with early support for his regime coming from the United Kingdom, Israel, and Apartheid-era South Africa.

In 1975–1976, Amin became the Chairman of the Organisation of African Unity, a pan-Africanist group designed to promote solidarity of the African states. During the 1977–1979 period, Uganda was appointed to the United Nations Commission on Human Rights. From 1977 to 1979, Amin titled himself as "His Excellency, President for Life, Field Marshal Al Hadji Doctor[B] Idi Amin Dada, VC,[C] DSO, MC, Conqueror of the British Empire in Africa in General and Uganda in Particular".

Dissent within Uganda and Amin's attempt to annex the Kagera province of Tanzania in 1978 led to the Uganda–Tanzania War and the demise of his regime. Amin fled first to Libya, then to Saudi Arabia, where he died in 2003.

Amin never wrote an autobiography nor did he authorise any official written account of his life. There are discrepancies regarding when and where he was born. Most biographical sources hold that he was born in either Koboko or Kampala around 1925.[A] According to Fred Guweddeko, a researcher at Makerere University, Idi Amin was the son of Andreas Nyabire (1889–1976). Nyabire, a member of the Kakwa ethnic group, converted from Roman Catholicism to Islam in 1910 and changed his name to Amin Dada. Abandoned by his father, Idi Amin grew up with his mother's family. Guweddeko states that Amin's mother was called Assa Aatte (1904–1970), an ethnic Lugbara and a traditional herbalist who treated members of Buganda royalty, among others. Amin joined an Islamic school in Bombo in 1941. After a few years, he left school with nothing more than a fourth grade education and did odd jobs before being recruited to the army by a British colonial army officer.

Amin joined the King's African Rifles (KAR) of the British Colonial Army in 1946 as an assistant cook. He claimed he was forced to join the Army during World War II and that he served in the Burma Campaign, but records indicate he was first enlisted after the war was concluded. He was transferred to Kenya for infantry service as a private in 1947 and served in the 21st KAR infantry battalion in Gilgil, Kenya, until 1949. That year, his unit was deployed to Somalia to fight the Somali Shifta rebels. In 1952 his brigade was deployed against the Mau Mau rebels in Kenya. He was promoted to corporal the same year, then to sergeant in 1953.

In 1959 Amin was made effendi (warrant officer), the highest rank possible for a Black African in the colonial British Army of that time. Amin returned to Uganda the same year and in 1961 he was promoted to lieutenant, becoming one of the first two Ugandans to become commissioned officers. He was then assigned to quell the cattle rustling between Uganda's Karamojong and Kenya's Turkana nomads. In 1962 he was promoted to captain and then, in 1963, to major. The following year, he was appointed Deputy Commander of the Army.

Amin was an active athlete during his time in the army. At 193 cm (6 ft 4 in) tall and powerfully built, he was the Ugandan light heavyweight boxing champion from 1951 to 1960, as well as a swimmer. Idi Amin was also a formidable rugby forward, although one officer said of him: "Idi Amin is a splendid type and a good (rugby) player, but virtually bone from the neck up, and needs things explained in words of one letter". In the 1950s, he played for Nile RFC. There is a frequently repeated urban legend that he was selected as a replacement by East Africa for their match against the 1955 British Lions. The story is entirely unfounded; he does not appear on the team photograph or on the official team list and replacements were not allowed in international rugby until 13 years after this event is supposed to have taken place.

In 1965 Prime Minister Milton Obote and Amin were implicated in a deal to smuggle ivory and gold into Uganda from the Democratic Republic of the Congo. The deal, as later alleged by General Nicholas Olenga, an associate of the former Congolese leader Patrice Lumumba, was part of an arrangement to help troops opposed to the Congolese government trade ivory and gold for arms supplies secretly smuggled to them by Amin. In 1966, Parliament demanded an investigation. Obote imposed a new constitution abolishing the ceremonial presidency held by Kabaka (King) Edward Mutesa II of Buganda, and declared himself executive president. He promoted Amin to colonel and army commander. Amin led an attack on the Kabaka's palace and forced Mutesa into exile to the United Kingdom, where he remained until his death in 1969.

Amin began recruiting members of Kakwa, Lugbara, Nubian, and other ethnic groups from the West Nile area bordering Sudan. The Nubians had been residents in Uganda since the early 20th century, having come from Sudan to serve the colonial army. Many ethnic groups in northern Uganda inhabit both Uganda and Sudan, therefore allegations persist that Amin's army consisted mainly of Sudanese soldiers.