A first cousin of the late Iraqi President Saddam Hussein, Al-Majid became one of Saddam's closest military advisors and head of the Iraqi Intelligence Service, Iraqi secret police known as the Mukhabarat. Following an unsuccessful assassination attempt on Saddam in 1983 in the town of Dujail, north of Baghdad, al-Majid directed the subsequent collective punishment operations in which scores of local men were killed, thousands more inhabitants were deported and the entire town was razed to the ground.
During the late stages of the Iran–Iraq War al-Majid was given the post of Secretary General of the Northern Bureau of the Ba'ath Party, in which capacity he served from March 1987 to April 1989. This effectively made him Saddam's proconsul in the north of the country, commanding all state agencies in the rebellious Kurdish-populated region of the country. He was known for his ruthlessness, ordering the indiscriminate use of chemical weapons such as mustard gas, sarin, tabun and VX against Kurdish targets during a genocidal campaign dubbed Al-Anfal, or The Spoils of War. The first such attacks occurred as early as April 1987 and continued into 1988, culminating in the notorious attack on Halabja in which over 5,000 people were killed.
With Kurdish resistance continuing, al-Majid decided to cripple the rebellion by eradicating the civilian population of the Kurdish regions. His forces embarked on a systematic campaign of mass killings, property destruction and forced population transfer (called "Arabization") in which thousands of Kurdish and Assyrian villages were razed and their inhabitants either killed or deported to the south of Iraq. He signed a decree in June 1987 stating that "Within their jurisdiction, the armed forces must kill any human being or animal present in these areas." By 1988, some 4,000 villages had been destroyed, an estimated 180,000 Kurds had been killed and some 1.5 million had been deported. The Kurds called him Chemical Ali ("Ali Kimyawi") for his role in the campaign; according to Iraqi Kurdish sources, Ali Hassan openly boasted of this nickname. Others dubbed him the "Butcher of Kurdistan".
Al-Majid was captured following the 2003 invasion of Iraq and was charged with war crimes, crimes against humanity and genocide. He was convicted in June 2007 and was sentenced to death for crimes of genocide against the Kurds committed in the al-Anfal campaign of the 1980s. His appeal of the death sentence was rejected on 4 September 2007, and he was sentenced to death for the fourth time on 17 January 2010 and was hanged eight days later, on 25 January 2010.