Soviet records show that Kim was born Yuri Irsenovich Kim (Russian: Юрий Ирсенович Ким) in 1941 in the village of Vyatskoye, near Khabarovsk, where his Father, Kim Il-sung, commanded the 1st Battalion of the Soviet 88th Brigade, made up of Chinese and Korean exiles. Kim Jong-il's mother, Kim Jong-suk, was Kim Il-sung's first wife. Inside his family, he was nicknamed "Yura", while his younger brother Kim Man-il (born Alexander Irsenovich Kim) was nicknamed "Shura".

However, Kim Jong-il's official biography states he was born in a secret military camp on Paektu Mountain (Chosŏn'gŭl: 백두산밀영고향집; Baekdusan Miryeong Gohyang jip) in Japanese-occupied Korea on 16 February 1942. According to one comrade of Kim's mother, Lee Min, word of Kim's birth first reached an army camp in Vyatskoye via radio and that both Kim and his mother did not return there until the following year.

In 1945, Kim was four years old when World War II ended and Korea regained independence from Japan. His Father returned to Pyongyang that September, and in late November Kim returned to Korea via a Soviet ship, landing at Sonbong. The family moved into a former Japanese officer's mansion in Pyongyang, with a garden and pool. Kim Jong-il's brother drowned there in 1948.

Reports indicate that his mother died in childbirth in 1949.

According to his official biography, Kim completed the course of general education between September 1950 and August 1960. He attended Primary School No. 4 and Middle School No. 1 (Namsan Higher Middle School) in Pyongyang. This is contested by foreign academics, who believe he is more likely to have received his early education in the People's Republic of China as a precaution to ensure his safety during the Korean War.

Throughout his schooling, Kim was involved in politics. He was active in the Korean Children's Union and the Democratic Youth League of North Korea (DYL), taking part in study groups of Marxist political theory and other literature. In September 1957 he became vice-chairman of his middle school's DYL branch (the chairman had to be a teacher). He pursued a programme of anti-factionalism and attempted to encourage greater ideological education among his classmates.

According to North Korean sources, Kim Jong-il published some 890 works during a period of his career from June 1964 to June 1994. According to KCNA, the number of works from 1964 to 2001 was 550. In 2000, it was reported that the Workers' Party of Korea Publishing House has published at least 120 works by Kim. In 2009, KCNA put the numbers as follows:

Kim's first wife, Hong Il-chon, was the daughter of a martyr who died during the Korean War. She was handpicked by his Father and married to him in 1966. They have a girl called Kim Hye-kyung, who was born in 1968. Soon, they divorced in 1969.

Kim's first mistress, Song Hye-rim, was a star of North Korean films. She was already married to another man and with a child when they met. Kim is reported to have forced her husband to divorce her. This relationship, started in 1970, was not officially recognized. They had one son, Kim Jong-nam (1971–2017), who was Kim Jong-il's eldest son. Kim kept both the relationship and the child a secret (even from his father) until he ascended to power in 1994. However, after years of estrangement, Song is believed to have died in Moscow in the Central Clinical Hospital in 2002.

Kim's official wife, Kim Young-sook, was the daughter of a high-ranking military official. His Father Kim Il-Sung handpicked her to marry his son. The two were estranged for some years before Kim's death. Kim had a daughter from this marriage, Kim Sul-song (born 1974).

Kim was said to be a huge film fan, owning a collection of more than 20,000 video tapes and DVDs. His reported favourite movie franchises included James Bond, Friday the 13th, Rambo, Godzilla and Hong Kong action cinema, with Sean Connery and Elizabeth Taylor his favourite male and female actors. He authored On the Art of the Cinema. In 1978, on Kim's orders South Korean film Director Shin Sang-ok and his Actress wife Choi Eun-hee were kidnapped in order to build a North Korean film industry. In 2006, he was involved in the production of the Juche-based movie The Schoolgirl's Diary, which depicted the life of a young girl whose parents are Scientists, with a KCNA news report stating that Kim "improved its script and guided its production".

After Ko's death, Kim lived with Kim Ok, his third mistress, who had served as his personal secretary since the 1980s. She "virtually act[ed] as North Korea's first lady" and frequently accompanied Kim on his visits to military bases and in meetings with visiting foreign dignitaries. She travelled with Kim Jong-il on a secretive trip to China in January 2006, where she was received by Chinese officials as Kim's wife.

The elder Kim had meanwhile remarried and had another son, Kim Pyong-il. Since 1988, Kim Pyong-il has served in a series of North Korean embassies in Europe and was the North Korean ambassador to Poland. Foreign commentators suspect that Kim Pyong-il was sent to these distant posts by his Father in order to avoid a power struggle between his two sons.

Kim's government was accused of "crimes against humanity" for its alleged culpability in creating and prolonging the 1990s famine.

On 24 December 1991, Kim was also named Supreme Commander of the Korean People's Army. Since the Army is the real foundation of power in North Korea, this was a vital step. Defence Minister Oh Jin-wu, one of Kim Il-sung's most loyal subordinates, engineered Kim Jong-il's acceptance by the Army as the next leader of North Korea, despite his lack of military Service. The only other possible leadership candidate, Prime Minister Kim Il (no relation), was removed from his posts in 1976. In 1992, Kim Il-sung publicly stated that his son was in charge of all internal affairs in the Democratic People's Republic.

The prevailing point of view is that the people's adherence to Kim Jong-il's cult of personality was solely out of respect for Kim Il-sung or out of fear of punishment for failure to pay homage. Media and government sources from outside North Korea generally support this view, while North Korean government sources aver that it was genuine hero worship. The song "No Motherland Without You", sung by the KPA State Merited Choir, was created especially for Kim in 1992 and is frequently broadcast on the radio and from loudspeakers on the streets of Pyongyang.

Kim was named Chairman of the National Defence Commission on 9 April 1993, making him day-to-day commander of the armed forces.

On 8 July 1994, Kim il-sung died at the age of 82 from a heart attack. Although Kim Jong-il had been his father's designated successor as early as 1974 and had been named commander-in-chief in 1991, it took him some time to consolidate his power.

In late March 2018, Reuters reported that Kim Jong-Il and his son Kim Jong-un both had fake passports supposedly issued by Brazil dated 26 February 1996, that were used to apply for visas in various countries. Both 10-year passports carry a stamp saying "Embassy of Brazil in Prague". Kim Jong-il's passport carries the name Ijong Tchoi and birth date of 4 April 1940 (actually in 1941). Kim Jong-un's passport has the name Josef Pwag and date of birth of 1 February 1983 (supposed to be 1982-84).

He officially took over his father's old post as General Secretary of the Workers' Party of Korea on 8 October 1997. In 1998, he was reelected as chairman of the National Defence Commission, and a constitutional amendment declared that post to be "the highest post of the state." Also in 1998, the Supreme People's Assembly wrote the president's post out of the constitution and designated Kim Il-sung as the country's "Eternal President" in order to honor his memory forever.

Kim was known as a skilled and manipulative diplomat. In 1998, South Korean President Kim Dae-jung implemented the "Sunshine Policy" to improve North-South relations and to allow South Korean companies to start projects in the North. Kim Jong-il announced plans to import and develop new technologies to develop North Korea's fledgling software industry. As a result of the new policy, the Kaesong Industrial Park was constructed in 2003 just north of the de-militarized zone.

United States Special Envoy for the Korean Peace Talks, Charles Kartman, who was involved in the 2000 Madeleine Albright summit with Kim, characterised Kim as a reasonable man in negotiations, to the point, but with a sense of humor and personally attentive to the people he was hosting. However, psychological evaluations conclude that Kim Jong-il's antisocial features, such as his fearlessness in the face of sanctions and punishment, served to make negotiations extraordinarily difficult.

In 2002, Kim Jong-il declared that "money should be capable of measuring the worth of all commodities." These gestures toward economic reform mirror similar actions taken by China's Deng Xiaoping in the late 1980s and early 90s. During a rare visit in 2006, Kim expressed admiration for China's rapid economic progress.

During Kim's rule, the country suffered from a famine and had a poor human rights record. Kim involved his country in state terrorism and strengthened the role of the military by his Songun, or "military-first", politics. Kim's rule also saw tentative economic reforms, including the opening of the Kaesong Industrial Park in 2003. In April 2009, North Korea's constitution was amended to officially refer to him (and his later successors) as the "supreme leader of the DPRK".

According to a 2004 Human Rights Watch report, the North Korean government under Kim was "among the world's most repressive governments", having up to 200,000 political prisoners according to U.S. and South Korean officials, with no freedom of the press or religion, political opposition or equal education: "Virtually every aspect of political, social, and economic life is controlled by the government."

In 1994, North Korea and the United States signed an Agreed Framework which was designed to freeze and eventually dismantle the North's nuclear weapons program in exchange for aid in producing two power-generating nuclear reactors and the assurance that it won't be invaded again. In 2000, after a meeting with Madeleine Albright, he agreed to a moratorium on missile construction. In 2002, Kim Jong-il's government admitted to having produced nuclear weapons since the 1994 agreement. Kim's regime argued the secret production was necessary for security purposes – citing the presence of United States-owned nuclear weapons in South Korea and the new tensions with the United States under President George W. Bush. On 9 October 2006, North Korea's Korean Central News Agency announced that it had successfully conducted an underground nuclear test.

Kim Yong Hyun, a political expert at the Institute for North Korean Studies at Seoul's Dongguk University, said in 2007: "Even the North Korean establishment would not advocate a continuation of the family dynasty at this point". Kim's eldest son Kim Jong-nam was earlier believed to be the designated heir but he appears to have fallen out of favor after being arrested at Narita International Airport near Tokyo in 2001 where he was caught attempting to enter Japan on a fake passport to visit Tokyo Disneyland.

In November 2008, Japan's TBS TV network reported that Kim had suffered a second stroke in October, which "affected the movement of his left arm and leg and also his ability to speak". However, South Korea's intelligence agency rejected this report.

On 9 April 2009, Kim was re-elected as chairman of the DPRK National Defence Commission and made an appearance at the Supreme People's Assembly. This was the first time Kim was seen in public since August 2008. He was unanimously re-elected and given a standing ovation.

There were speculations that the visits of Kim Jong-il abroad in 2010 and 2011 were a sign of his improving health and a possible slowdown in succession might follow. After the visit to Russia, Kim Jong-il appeared in a military parade in Pyongyang on 9 September, accompanied by Kim Jong-un.

It was reported that Kim Jong-il had died of a suspected heart attack on 17 December 2011 at 8:30 a.m. while travelling by train to an area outside Pyongyang. It was reported in December 2012, however, that he had died "in a fit of rage" over construction faults at a crucial power plant project at Huichon in Jagang Province. He was succeeded by his youngest son Kim Jong-un, who was hailed by the Korean Central News Agency as the "Great Successor". According to the Korean Central News Agency (KCNA), during his death a fierce snowstorm "paused" and "the sky glowed red above the sacred Mount Paektu" and the ice on a famous lake also cracked so loud that it seemed to "shake the Heavens and the Earth."

In February 2012, on what would have been his 71st birthday, Kim Jong-il was posthumously made Dae Wonsu (usually translated as Generalissimo, literally Grand Marshal), the nation's top military rank. He had been named Wonsu (Marshal) in 1992 when North Korean founder Kim Il-sung was promoted to Dae Wonsu. Also in February 2012, the North Korean government created the Order of Kim Jong-il in his honor and awarded it to 132 individuals for services in building a "thriving socialist nation" and for increasing defense capabilities.

In his teens and university years, Kim Jong-il wrote poems – notably "O Korea, I will Add Glory to Thee". Kim Jong-il also wrote song lyrics. His first major literary work was On the Art of the Cinema in 1973.