He became king on the death of his father, King Cyprian Bhekuzulu kaSolomon, in 1968. Prince Israel Mcwayizeni kaSolomon acted as the regent from 1968 to 1971 while the King took refuge in St. Helena for three years to avoid assassination. After his 21st birthday and his first marriage, Zwelithini was installed as the eighth monarch of the Zulus at a traditional ceremony at Nongoma on 3 December 1971, attended by 20,000 people.
In the power vacuum created in the 1990s as Apartheid and the domination of the country by White South Africans was abolished, the King was sometimes unable to avoid being drawn into partisan politics. The Zulu-dominated Inkatha Freedom Party (IFP) initially opposed parts of the new constitution advocated by the African National Congress (ANC) regarding the internal governance of KwaZulu. In particular, the IFP campaigned aggressively for an autonomous and sovereign Zulu king, as constitutional head of state. As a result, the IFP abstained from registering its party for the 1994 election (a necessity in order to receive votes) in opposition. However, once it became obvious that its efforts were not going to stop the election (the IFP's desired goal), the party was registered. It demonstrated its political strength by taking the majority of the provincial votes for KwaZulu-Natal.
Although the constitution makes the role of the King largely ceremonial, and it is incumbent upon him to act on the official advice of the provincial premier, on occasion South African President Nelson Mandela made efforts to bypass the IFP in negotiating with the Zulus, instead making direct overtures to the King (Mandela's daughter, Zeni, is married to Prince Thumbumuzi Dlamini, a brother of Zwelithini's "Great Wife", Queen Mantfombi). Nonetheless, the IFP remained in power in the province until 2003.
During most of the King's reign his cousin (uncle in Zulu African reckoning), Mangosuthu Buthelezi, Prince of KwaPhindangene and founder/head of IFP, was the Zulu prime minister. But in September 1994 tension between the previously allied kinsmen peaked publicly as the annual Shaka Zulu celebration approached. Rumors that the King was maneuvering to replace Buthelezi as Zulu prime minister with former regent Prince Mcwayizeni, who had joined the ANC in 1990, seemed likely after the King announced that Buthelezi would no longer be his chief advisor, and simultaneously cancelled the holiday ceremony. For his safety, federal troops escorted Zwelithini by helicopter to Johannesburg. Although Buthelezi was then serving as Home Affairs minister in South Africa's Cabinet, President Mandela's efforts to broker a reconciliation failed. Buthelezi moved the event from Nongoma to Stanger, and addressed a throng of 10,000 of his Zulu supporters.
Subsequently, the King's spokesman, Prince Sifiso Zulu, was being interviewed on television at the South African Broadcasting Corporation's studio when Buthelezi and his bodyguards forcibly interrupted the programme, physically intimidating Chief Sifiso. The televised incident drew national attention and a public rebuke from Mandela, prompting Buthelezi to apologize to the Zulu Royal Family, Cabinet and nation for his behavior. Relations between Zwelithini and Buthelezi later improved.
King Zwelithini has cooperated as the law requires with the ANC since it took over the reins of government in KwaZulu-Natal. The King's finances are controlled by KwaZulu-Natal provincial authorities.
In 1989 he criticized the ANC leadership for not inviting him and Buthelezi to a rally welcoming back the Rivonia Trial defendants, who had been released after almost three decades of imprisonment.
As the constitutional monarch of the kingdom of KwaZulu-Natal, he is head of the Ubukhosi, the state-recognized institution of Traditional Leadership that consists of local chiefs. His leadership role also entails chairmanship of the Usuthu Tribal Authority and Nongoma Regional Authority, both established under the provisions of the KwaZulu Amakhosi and Iziphakanyiswa Act. In his address upon the opening of the Provincial Parliament on September 28, 2003, the King advised the government and legislators to give more heed to the Ubukhosi:
Traditional Leaders are neither consulted nor involved in the process of formulating policies that have a direct bearing on their day to day activities. The institution of Ubukhosi has been in existence from time immemorial and has survived many hardships under past colonial regimes. From the point of view of the ordinary citizen, an Inkosi's most important role may lie in his symbolizing of community solidarity. So any notion that the institution of Ubukhosi, now that we have a democratic government in place, can just be wished away, remains a pipe-dream. Some countries just across our borders had decided to do away with the institution of traditional leadership immediately after attaining independence from colonial rulers. However, they have since realised that they had committed gross mistakes and were now re-inventing these institutions at great costs. As King of the Zulu Nation I am proud of the role played by the Prime Minister of the Zulu Nation, Prince of KwaPhindangene, Dr MG Buthelezi who had singlehandedly championed the cause of the Institution of Traditional Leadership in this country.
Zwelithini has been criticised for buying luxury Mercedes Benz motor cars and other expensive vehicles for his wives when a large percentage of the population in Kwa-Zulu Natal is living in poverty.
The King is chairman of the Ingonyama Trust, a corporate entity established to administer the land traditionally owned by the king for the benefit, material welfare and social well-being of the Zulu nation. This land consists of 32% of the area of KwaZulu/Natal.
As the custodian of Zulu traditions and customs, King Zwelithini has revived cultural functions such as the Umhlanga, the colourful and symbolic reed dance ceremony which, amongst other things, promotes moral awareness and AIDS education among Zulu women, and the Ukweshwama, the first fruits ceremony, which is a traditional function involving certain traditional rituals including the killing of a bull. The latter ceremony was subject to a lawsuit brought in November 2009 by Animal Rights Africa, alleging that the method of killing the animal was cruel and barbaric. He has also traveled abroad extensively to promote tourism and trade in the West for KwaZulu-Natal, and to fundraise for Zulu-supported charities, often accompanied by one of his queens consort. On such occasions he is frequently officially hosted by local Zulu organizations, and grants audiences to Zulus living abroad.
In June, 1994, the University of Zululand conferred an honorary doctorate in agriculture upon the King. He is Chancellor of the South African branch of the American-based Newport University. In March 1999 Coker College of South Carolina awarded him an honorary doctorate in law. During the first half of 2001 he was inaugurated as Chancellor of the M L Sultan Technikon in KwaZulu-Natal.
The King's authorized biography, King of Goodwill, was published in 2003. The musical dramatization of this work premiered at the Market Theatre, Johannesburg on 16 March 2005.
The King spoke at The Synagogue Church Of All Nations in Lagos, Nigeria, in 2004 regarding the importance of trade and peace.
In January, 2012, while speaking at an event commemorating the 133rd anniversary of the Battle of Isandlwana, the King caused controversy with his statement that same-sex relations were "rotten". His statements were condemned by the South African Human Rights Commission as well as LGBT rights groups. President Jacob Zuma rebuked the king for his comments. The Zulu Royal Household later said that the King's comments had been mistranslated and that he had not condemned same-sex relations, only expressed concern about a state of moral decay in South Africa that has led to widespread sexual abuse, including male-on-male sexual abuse.
In September 2012, King Goodwill Zwelithini asked the KwaZulu-Natal government for R18m to build new property, including a new R6m palace for his youngest wife Queen Mafu and upgrades to Queen MaMchiza's palace. The King's royal household department CFO, Mduduzi Mthembu, told a parliamentary committee that the money was needed. The department also requested USD1.4m for improvements to Queen MaMchiza's palace. The government had already budgeted around USD6.9m for the royal family during 2012, not for the first time prompting accusations of lavish spending; in 2008, opposition parties criticised King Zwelithini's wives for spending around USD24,000 on linen, designer clothes and expensive holidays.
Speaking at a Pongolo community meeting in March 2015, Zwelithini acknowledged that while other nations had participated in efforts which led to South Africa's liberation, that should not be considered an excuse for foreigners to cause inconvenience in the country now by competing with locals for scarce economic opportunities. Contending that he was free to say what politicians were not, he asked that foreigners please return to their native lands since, he maintained, South African nationals in diaspora had not gone on to open businesses in host countries. These observations were made during a time of growing unease between South Africans and non-nationals, violence having erupted in Soweto in January and spread to KwaZulu-Natal, resulting in three deaths. The Democratic Alliance's spokesman, calling for a public retraction and apology, criticised the remarks as "highly irresponsible", while a SAHRC official labelled them xenophobic in light of recent attacks on foreigners. Alleged to have sparked violence against non-nationals, although Zwelithin's remarks about anti-social behaviour and the desirability of foreigners believed responsible leaving South Africa did not distinguish between legal and illegal immigrants, his spokesman subsequently said that he was referring only to those present in the country illegally.
27 children (as of 2003), including:
- maDlamini (born Sibongile Winifred Dlamini), married 27 December 1969] at St Margaret's Church, Nongoma.
- Prince Lethukuthula Zulu (by Queen Sibongile MaDlamini), born 1970.
- Princess Nombuso Zulu (by Queen Sibongile MaDlamini) owner of Durban-based, Ilembe Catering Services,born 1973.
- Ntombizosuthu Ka Zwelithini Duma (by Queen Sibongile MaDlamini) a businesswoman who co-owns Strategic Persuasions and Zamalwandle Transport Logistics with her husband. Born 1979, Married to Mbongiseni Duma, a Johannesburg-based business man.
- Princess Ntandoyenkosi Ka Zwelithini Ngcaweni (by Queen Sibongile MaDlamini), an Asset Manager at the Public Investment Corporation(PIC), born 1982. Married to Busani Ngcaweni, who heads the office of the Deputy President of the Republic Of South Africa, Deputy President Kgalema Motlante.
- Princess Snethemba Bati Zulu (by Queen Sibongile Dlamini), born 1989, currently pursuing a degree in International Relations, at the University of Witwatersrand.
- Buhle KaMathe, born c1951. In May 1996, she and her daughter were seriously wounded in an assault during which they were clubbed, stabbed and shot.
- Princess Sibusile Zulu (by Queen Buhle KaMathe), born 1972.
- Princess Nandi Zulu (by Queen Buhle KaMathe), born 1977, married (civil) 6 December 2002 in St John's Cathedral, Mthatha by Bishop Sitembele Mzamane in Mthatha and (traditional) 7 December at the Thembu Great Place near Qunu, to Chief Mfundo Bovulengwa Mtirara, born 25 March 1973, Acting Deputy Paramount Chief of the Thembu from 2000, Chief of the Matye'ngqina Traditional Authority Area.
- Prince Phumuzuzulu (by Queen Buhle KaMathe - Phumuza, named after his Great Grandfather King Phumuzululu kaDinuzulu, son King kaCetshwayo
- Prince Shlobosenkosi Zulu (by Queen Buhle KaMathe) born 1988 is currently an 11th Grade student at Kearsney College in Botha's Hill, Durban.
- Mantfombi Dlamini, born 1956, daughter of Sobhuza II of Swaziland and sister of Mswati III, married 1973.
- Prince Misuzulu Zulu (by Queen Mantfombi), born 23 September 1974 in Kwahlabisa, KwaZulu-Natal, is currently pursuing a degree in International Studies in Jacksonville, Florida, and is a strong candidate for Zwelethini's successor. He is unmarried and has one son.
- Princess Ntandoyesizwe Zulu (by Queen Mantfombi), born 1976, married 13 April 2002 at Enyokeni Royal Palace, Nongoma, to Kgosi Oupa Moilwa, Chief of the Bahurutse Bagamoilwa. Civil ceremony July 11, 2004 in Pongola.
- Princess Lomkhosi (by Queen Mantfombi), born 1982, fiancee Melusi Moyo
- Princess Bukhosibemvelo, (by Queen Mantfombi), born 1985, married Sipho Nyawo, who paid 120 cows as part of ilobolo for the Zulu princess.
- Prince Lungelo (by Queen Mantfombi), a student at Michaelhouse boarding school
- Prince Mandlesizwe (by Queen Mantfombi)
- Prince Bizwekhaya (by Queen Mantfombi)
- Prince Masikomahle (by Queen Mantfombi)
- Thandi (born Thandekile Jane Ndlovu).
- Nompumelelo Mchiza, married 25 July 1992.
- Zola Zelusiwe Mafu, born c 1986, betrothed 2006, married 2014.
- Prince Nhlendla (by Queen LaMafu)