On 31 March 2005, following consultation with the Crown Council of Monaco, the Palais Princier announced that Albert would take over the duties of his father as regent since Rainier was no longer able to exercise his royal functions. On 6 April 2005, Rainier died and Albert succeeded him as Albert II.
The first part of Prince Albert II's enthronement as ruler of the Principality was on 12 July 2005, after the end of the three-month mourning period for his father. A morning Mass at Saint Nicholas Cathedral presided over by the Archbishop of Monaco, the Most Reverend Bernard Barsi, formally marked the beginning of his reign. Afterward, Albert returned to the Palace to host a garden party for 7,000 Monégasques born in the principality. In the courtyard, the Prince was presented with two keys of the city as a symbol of his investiture, and subsequently gave a speech. The evening ended with a fireworks display on the waterfront.
The second part of his investiture took place on 19 November 2005. Albert was enthroned at Saint Nicholas Cathedral. The Princely family was in attendance, including his elder sister, Princess Caroline with her husband Ernst, Prince of Hanover and three of her four children, Andrea, Pierre and Charlotte; as well as his younger sister Princess Stéphanie, his paternal aunt Princess Antoinette, Baroness of Massy, his godson, Jean-Léonard Taubert-Natta de Massy, and his cousin Elisabeth-Anne de Massy. Royalty from 16 delegations were present for the festivities throughout the country. The evening ended with an dedicated performance at the Opéra de Monte-Carlo.
In the early years of his reign, Prince Albert oversaw multiple judicial and legal reforms, including the regulation of custody, protections of the privacy of the individual in the face of technological growth, freedom of the press, legislative gender equality, and the protection of children's rights and handicapped students. In July 2005, in echo of Albert I, his great-great-grandfather, he travelled to Spitsbergen, Norway. During this trip, he visited the glaciers Lilliehöökbreen and Monacobreen. Prince Albert also engaged in a Russian Arctic expedition, reaching the North Pole on Easter, 16 April 2006. He is the first incumbent head of state to have reached the North Pole.
In October 2005, the German magazine Bunte reported that Prince Albert was dating Telma Ortiz Rocasolano, a sister-in-law of the Prince of Asturias (who is now the King of Spain). However, in November 2005, the Prince instructed his lawyer, Thierry Lacoste, to commence legal proceedings against the French newspaper France Dimanche for violation of privacy and false information regarding the story.
In May 2005, Nicole Coste, a former Air France flight attendant from Togo, claimed that her youngest son, whom she named Alexandre Coste, was Prince Albert's child, and stated that his parentage had been proven by DNA tests requested by the Monegasque government. She further declared that the Prince had signed a notarised certificate confirming paternity, which she had not received a copy of. TheParis Match published a ten-page interview with Coste, including photographs of Albert holding and feeding the child. Coste also told the publication that she was living in the Prince's Paris apartment, and receiving an allowance from him, while pretending to be the girlfriend of one of his friends in order to maintain discretion. She also stated that the prince had previously last seen the boy in February 2005. The prince's lawyer, Thierry Lacoste, announced that as a result of the international publicity over these revelations, Prince Albert was suing the Daily Mail, Bunte, and Paris Match for privacy violations. On 6 July 2005, a few days before he was enthroned on 12 July, the Prince officially confirmed via his lawyer Lacoste that Alexandre was his biological son.
Under the current constitution, neither Jazmin nor Alexandre are in the line of succession to the Monegasque throne as they are not Prince Albert's legitimate children, and he emphasised their ineligibility to inherit the throne in statements confirming his paternity. Monegasque law stipulates that any non-adulterine illegitimate child is legitimised by the eventual marriage of his/her parents, thereupon obtaining the rights to which that child would have been entitled if born in lawful marriage. Thus Alexandre would have become Monaco's heir apparent under current law if Albert were to marry his son's mother. But in a 2005 exchange with American reporter Larry King, Albert stated that this would not happen.