[8] Hun Sen rose to the premiership in January 1985 when the one-party National Assembly appointed him to succeed Chan Sy, who had died in office in December 1984.

Between 1998 and 2023, Hun Sen led the CPP to consecutive and often contentious election victories, overseeing rapid economic growth and development, but also corruption, deforestation and human rights violations.

In 2018, he was elected to a sixth and final term in a largely unopposed poll after the Supreme Court dissolved the main opposition party, with the CPP winning every seat in the National Assembly.

[14] He led the country during the onset of the COVID-19 pandemic and Cambodia's third chairmanship of ASEAN; and, after the 2023 election formally announced his resignation as prime minister in favour of his son, Hun Manet.

[26] Sen rapidly ascended ranks as a soldier, and fought during the fall of Phnom Penh, becoming injured and being hospitalized for some time[7] and sustaining a permanent eye injury.

[7][8][33] Human Rights Watch suggested he may have had a role in a massacre to suppress Cham Muslim unrest in September–October 1975, but Sen has denied this, claiming that he had stopped following orders from the central government by this time.

[8] The Vietnamese-appointed government appointed Sen some authority over the K5 Plan, a Khmer Rouge containment strategy that saw the mass mobilization of civilian labor in constructing barricades and land mines, although the extent of his involvement is unclear.

[7] Hun Sen first rose to the premiership in January 1985 when the one-party National Assembly encouraged by politburo cadre Say Phouthang[34] appointed him to succeed Chan Sy, who had died in office in December 1984.

[7] In 1987, Amnesty International accused Hun Sen's government of torturing thousands of political prisoners, using "electric shocks, hot irons and near-suffocation with plastic bags.

He held the position of prime minister during the United Nations Transitional Authority in Cambodia (UNTAC) until the 1993 UN-sponsored elections, which resulted opposition party FUNCINPEC winning the majority of votes with a hung parliament.

This deadlock was overcome when a new CPP-FUNCINPEC coalition was formed in mid-2004, with Norodom Ranariddh chosen to be head of the National Assembly and Hun Sen again becoming sole prime minister.

He ordered the military to "attack" them on sight should they return,[67] threatened airlines with legal actions for allowing them to board, deployed thousands of troops to the Thai and Vietnamese borders, and requested other ASEAN leaders arrest them and deport them to Cambodia.

[68] In 2020, the European Union suspended its Everything but Arms preferential trade agreement with Cambodia due to concerns over human rights violations under Hun Sen's government.

[69] During the early stages of the COVID-19 pandemic, Hun Sen downplayed the risk of the virus and declined to introduce preventative measures or evacuate Cambodian citizens from Wuhan during the initial outbreak in China.

[74] A new State of Emergency Law prepared in response to COVID-19 granted Hun Sen further powers to restrict movement and assembly, seize private property and enforce quarantine.

[77] Hun Sen and his family were estimated to have amassed between US$500 million and US$1 billion by Global Witness in 2016,[78] and a number of allies have also accumulated considerable personal wealth during his tenure.

He and his close associates were accused of carrying out secret negotiations with interested private parties, taking money from those[further explanation needed] who would be granted rights to exploit the country's resources in return.

[42] Hun Sen frequently calls for violence against his political opponents during seemingly irrelevant public events, often characterizing this as necessary to maintain peace and stability in Cambodia.

[90] In 2019, as opposition party leaders prepared to return to the country, Sen ordered the military to "attack them wherever you see them—you don't need arrest warrants at all" while speaking at a graduation ceremony for exceptional high school students in Phnom Penh.

[94][24] Several Australian politicians, most prominently Gareth Evans and Julian Hill, have been highly critical of Sen and his government over human rights issues and have called for changes to Australia–Cambodia relations.

[98] Sen has frequently criticized Western powers such as the European Union and United States in response to their sanctions on Cambodia over human rights issues.

[69][99] Sen strengthened a close diplomatic and economic relationship with China, which has undertaken large-scale infrastructure projects and investments in Cambodia under the Belt and Road Initiative.

[17] Hun Sen's leadership has received criticism from various organizations, media and foreign governments for corruption, cronyism, environmental degradation, human rights violations and violence.

[115] Norodom Sihanouk once referred to him as a "one-eyed lackey of the Vietnamese",[115] with Sam Rainsy and members of the Cambodia National Rescue Party later echoing similar sentiments during the 2010s.

The country's most prominent independent newspaper Cambodia Daily was closed on 4 September 2017, a day after the main opposition leader Kem Sokha was arrested for treason.

[45][20][126][127][128] Cambodia has also prosecuted women who post images of themselves wearing revealing clothing on Facebook, with Hun Sen saying it is "a violation of culture and tradition" and invites sexual harassment.

[131][129] On 29 June 2023, Hun Sen deleted his Facebook account, which had approximately 14 million followers, hours after Meta's oversight board ruled that he should face a six-month ban from the platform over a video post in which he threatened to have opponents beaten.

[132][133][134][135] On the following day, 30 June 2023, the Cambodian Ministry of Post and Telecommunications announced they would deport a Meta representative immediately and Cambodia would cease all cooperation with the company, attributing the move to an abundance of fake accounts, data risks, and lack of transparency.

[138][139] In 2010, Manet was promoted Major General in the Royal Cambodian Armed Forces (RCAF) and became the Deputy Commander of the Prime Minister's Body Guard headquarters.

Hun Sen also speaks some English after beginning to learn the language in the 1990s, but usually converses in Khmer through interpreters when giving formal interviews to the English-speaking media.