North Korean media were describing the economy as a "flexible collectivist system" where enterprises were applying "active and evolutionary actions" to achieve economic development. These reports reflect Kim's general economic policy of reforming management, increasing the autonomy and incentives for economic actors. This set of reforms known as the "May 30th measures" reaffirms both socialist ownership and "objective economic laws in guidance and management" to improve living standards. Other objectives of the measures are to increase the availability of domestically manufactured goods on markets, introduction of defence innovations into the civilian sector and boost international trade.

Scarce information on Kim Jong-un's early life comes from North Korean defectors and people who have claimed to witness him abroad, such as during his school attendance in Switzerland. Some of the information has been conflicting and contradictory, perhaps confusing him with his brother, Kim Jong-chul, who also attended school in Switzerland around the same time. North Korean authorities have stated that his birthdate is 8 January 1982, but South Korean intelligence officials believe the actual date is a year later. Former basketball star Dennis Rodman said that the birthdate is 8 January 1983 after meeting Kim in September 2013 in North Korea. Kim Jong-Un was the second of three children Ko Yong-hui bore to Kim Jong-il; his elder brother Kim Jong-chul was born in 1981, while his younger sister, Kim Yo-jong, is believed to have been born in 1987.

According to reports first published in Japanese newspapers, he went to school in Switzerland near Bern. First reports claimed he attended the private English-language International School in Gümligen under the name "Chol-pak" or "Pak-chol" from 1993 to 1998. He was described as shy, a good student who got along well with his classmates and was a basketball fan. He was chaperoned by an older student, who was thought to be his bodyguard.

In late February 2018, Reuters reported that Kim and his father had used forged passports—supposedly issued by Brazil and dated 26 February 1996—to apply for visas in various countries. Both 10-year passports carry a stamp saying "Embassy of Brazil in Prague". Kim Jong-un's passport records the name "Josef Pwag" and a date of birth of 1 February 1983.

It is believed that the student at the Gümligen International School may not have been Kim Jong-un, but his elder brother Kim Jong-chul. It is not known whether the student known as Pak-un in Liebefeld Steinhölzli lived in Switzerland prior to 1998. All the children of Kim Jong-il are said to have lived in Switzerland, as well as the mother of the two youngest sons, who lived in Geneva for some time.

The Laboratory of Anatomic Anthropology at the University of Lyon, France, after comparing the picture of the boy Pak-un taken at the Liebefeld Steinhölzli school in 1999 with a picture of Kim Jong-un from 2012 came to the conclusion that the two faces show a conformity of 95%. The head of the institute, Raoul Perrot, a forensic Anthropologist, considers it most likely that the two pictures show the same person.

Kim Jong-un's eldest half-brother, Kim Jong-nam, had been the favorite to succeed, but reportedly fell out of favor after 2001, when he was caught attempting to enter Japan on a fake passport to visit Tokyo Disneyland. Kim Jong-nam was killed in Malaysia in 2017 by suspected North Korean agents.

Most analysts agree that Kim Jong-un attended Kim Il-sung University, a leading officer-training school in Pyongyang, from 2002 to 2007.

In 2009, reports suggested that Kim Jong-un was a diabetic and suffered from hypertension. He is also known to smoke cigarettes.

On 10 October 2010, alongside his father, Kim Jong-un attended the ruling Workers' Party's 65th anniversary celebration. This was seen as confirming his position as the next leader of the Workers' Party. Unprecedented international press access was granted to the event, further indicating the importance of Kim Jong-un's presence. In January 2011, the regime reportedly began purging around 200 protégés of both Jong-un's uncle-in-law Jang Song-thaek and O Kuk-ryol, the vice chairman of the National Defence Commission, by either detention or execution to further prevent either man from rivaling Jong-un.

On 26 December 2011, the leading North Korean newspaper Rodong Sinmun reported that Kim Jong-un had been acting as chairman of the Central Military Commission, and supreme leader of the country, following his father's demise.

On 25 July 2012, North Korean state media reported for the first time that Kim Jong-un is married to Ri Sol-ju (리설주). Ri, who appears to be in her early 20s, had been accompanying Kim Jong-un to public appearances for several weeks prior to the announcement. According to a South Korean analyst, Kim Jong-il had hastily arranged the marriage after suffering a stroke in 2008, the two married in 2009, and they had a child in 2010. Dennis Rodman, after visiting in 2013, reported that they had a daughter named Ju-ae. However, South Korean sources speculated that they could have many children.

The Washington Post reported in 2009 that Kim Jong-un's school friends recalled he "spent hours doing meticulous pencil drawings of Chicago Bulls superstar Michael Jordan". He was obsessed with basketball and computer games. On 26 February 2013, Kim Jong‑un met Dennis Rodman, leading many reporters to speculate that Rodman was the first American that Kim had met. Rodman described his trip to Kim Jong-un's private island, "It's like Hawaii or Ibiza, but he's the only one that lives there." Kim Jong-un’s net worth is estimated at US$5 billion.

Kim Jong-un did not appear in public for six weeks in September and October 2014. State media reported that he was suffering from an "uncomfortable physical condition". Previously he had been seen limping. When he reappeared, he was using a walking stick.

In September 2015, the South Korean government commented that Kim appeared to have gained 30 kg in body fat over the previous five years, reaching a total estimated body weight of 130 kg (290 lb).

In July 2016, the United States Department of the Treasury imposed personal sanctions on Kim. Although his involvement in human rights abuses was cited as the reason, officials said the sanctions target the country's nuclear and missile programs.

On 13 February 2017, Kim Jong-nam, the exiled half-brother of Kim Jong-un, was assassinated with the nerve agent VX while walking through Terminal 2 at Kuala Lumpur International Airport.

In his 2018 New Year Speech, Kim said that he was open to dialogue with South Korea with a view to take part in the upcoming Winter Olympics in the South. The Seoul–Pyongyang hotline was reopened after almost two years. North and South Korea marched together in the Olympics opening ceremony and fielded a united women's ice hockey team. In addition to the athletes, Kim sent an unprecedented high-level delegation headed by his sister, Kim Yo-jong, and de facto head of state Kim Yong-nam, including performers such as the Samjiyon Orchestra. On 5 March, he arranged a meeting with South Korea's Chief of the National Security Office, Chung Eui-yong. Chung then traveled to Washington and passed on an invitation to U.S. President Donald Trump to meet with Kim, which Trump accepted.

Kim is sometimes accompanied by his younger sister Kim Yo-jong, who is said to be instrumental in creating his public image and organising public events for him. According to Kim Yong-hyun, a professor of North Korean studies at Dongguk University in Seoul, and others, the promotion of Kim Yo-jong and others is a sign that "the Kim Jong-un regime has ended its co-existence with the remnants of the previous Kim Jong-il regime by carrying out a generational replacement in the party’s key elite posts".