Juan Carlos was expected to continue Franco's legacy, but instead introduced reforms to dismantle the Francoist regime and to begin the Spanish transition to democracy soon after his accession.

He was baptized as Juan Carlos Alfonso Víctor María de Borbón y Borbón-Dos Sicilias by Cardinal Eugenio Pacelli, the future Pope Pius XII.

[11] On the evening of Holy Thursday, 29 March 1956, Infante Alfonso died in a gun accident at the family's home Villa Giralda in Estoril, on the Portuguese Riviera.

The Spanish Embassy in Portugal then issued the following official communiqué:[12] Whilst His Highness Prince Alfonso was cleaning a revolver last evening with his brother, a shot was fired hitting his forehead and killing him in a few minutes.

The accident took place at 20.30 hours, after the Infante's return from the Maundy Thursday religious service, during which he had received holy communion.Alfonso had won a local junior golf tournament earlier in the day, then went to evening Mass and rushed up to the room to see Juan Carlos who had come home for the Easter holidays from military school.

However, as the years progressed, Juan Carlos began meeting secretly with political opposition leaders and exiles, who were fighting to bring liberal reform to the country.

Franco, for his part, remained largely oblivious to the prince's actions and denied allegations from his ministers and advisors that Juan Carlos was in any way disloyal to his vision of the regime.

In his address to the Cortes, Juan Carlos spoke of three factors: historical tradition, national laws, and the will of the people, and in so doing referred to a process dating back to the Civil War of 1936–39.

[21] He swore using the following formula: "I swear to God and the Gospels to comply and enforce compliance to the Fundamental Laws of the Realm and to remain loyal to the Principles of the National Movement".

[21][29] Juan Carlos is reported to have been pressured by Valéry Giscard d'Estaing to personally tell Chilean dictator Augusto Pinochet, who had traveled to Spain for Franco's funeral, not to attend his inauguration.

[34] Further legitimacy was restored to Juan Carlos's position on 14 May 1977, when his father (whom many monarchists had recognized as the legitimate, exiled King of Spain during the Franco era) formally renounced his claim to the throne and recognized his son as the sole head of the Spanish Royal House, transferring to him the historical heritage of the Spanish monarchy, thus making Juan Carlos both de facto and de jure king in the eyes of the traditional monarchists.

[35] On 20 May 1977, the leader of the only recently legalized Spanish Socialist Workers' Party (PSOE), Felipe González, accompanied by Javier Solana, visited Juan Carlos in the Zarzuela Palace.

[36] Left-wing support for the monarchy had grown when the Communist Party of Spain was legalized on 9 April 1977, a move Juan Carlos had pressed for, despite enormous right-wing military opposition at that time, during the Cold War.

During the coup, the King, wearing his uniform as Captain-General of the Armed Forces, gave a public television broadcast calling for unambiguous support for the legitimate democratic government.

Above all, Juan Carlos and the main political parties were aware of a plan to put General Alfonso Armada in charge of the government, particularly in order to crack down on the Basque independence organization Euskadi ta Askatasuna (ETA).

Although Juan Carlos strongly condemned the coup attempt — more than six hours after the armed guards invaded Congress — it is still difficult to establish whether he acted out of democratic conviction or because the operation was not going as well as expected, with little support.

As head of the Spanish state, Juan Carlos "held political power, gave his opinion and exerted his influence in the economic sphere, for example, in the area of company mergers or public policy during the transition period," analyses journalist Ana Pardo.

[49] In November 2007, at the Ibero-American Summit in Santiago, during a heated exchange, Juan Carlos interrupted Venezuelan President Hugo Chávez, saying, "¿Por qué no te callas?"

[66] Spanish news media started to speculate about the King's future in 2013, following public criticism over his taking an elephant hunting safari in Botswana and an embezzlement scandal involving his daughter, Infanta Cristina, Duchess of Palma de Mallorca, and her husband Iñaki Urdangarin.

Juan Carlos was the fourth European monarch to abdicate in just over a year, following Pope Benedict XVI (28 February 2013), Queen Beatrix of the Netherlands (30 April 2013), and King Albert II of Belgium (21 July 2013).

[77] However, unlike his previous immunity, the new legislation left him accountable to the supreme court, in a similar type of protection afforded to many high-ranking civil servants and politicians in Spain.

[84] British Prime Minister David Cameron stated: "I would like to use this opportunity to make a tribute to King Juan Carlos, who has done so much during his reign to aid the successful Spanish transition to democracy, and has been a great friend of the United Kingdom.

[100] Sayn-Wittgenstein claimed that Juan Carlos received kick-backs from commercial contracts in the Gulf States – particularly in the late-2000s construction of the €6.7 billion Haramain high-speed railway in Saudi Arabia – and maintained these proceeds in a bank account in Switzerland.

[103][104] Sayn-Wittgenstein reportedly told the head Swiss prosecutor on 19 December 2018 that Juan Carlos had given her €65 million out of "gratitude and love", to guarantee her future and her children's, because "he still had hopes to win her back".

[115] Mexican millionaire and investment banker Allen Sanginés-Krause has been named as the owner of the cards, a friend of Juan Carlos to whom he donated sums of money using Air Force Colonel Nicolás Murga Mendoza as an intermediary.

[116] In December 2020, Juan Carlos reportedly paid 678,393.72 euros to Spain's tax agency for the concept of defrauded money in an affair of "opaque credit cards" used between 2016 and 2018 by himself, his wife and some grandchildren, intending to avoid further scrutiny from the Supreme Court's prosecutor, the payment being an admission of fraud.

[134] On 3 August 2020, the Royal Household announced Juan Carlos wished to relocate from Spain because of increased media press about his business dealings in Saudi Arabia, and he had left a letter to his son saying so.

During their summer holidays, the whole family spends time at Marivent Palace (Palma de Mallorca) and on the yacht Fortuna, where they would take part in sailing competitions.

[161] A benign 17–19 mm tumour was removed under general anaesthetic from King Juan Carlos's right lung in an operation carried out in the Hospital Clínic de Barcelona in May 2010.

[164] In April 2012, the King underwent surgery for a triple fracture of the hip at the San Jose Hospital, Madrid, following a fall on a private elephant-hunting trip to Botswana.