Iliescu's father, Alexandru Iliescu, was a railroad worker with Communist views during the period in which the Romanian Communist Party was banned by the authorities. In 1931, he went to the Soviet Union to take part in the Communist Party Congress of Moscow. He remained in the USSR for the next four years and was arrested upon his return. He was imprisoned from June 1940 to August 1944 and died in August 1945. During his time in the Soviet Union, Alexandru Iliescu divorced and married Marița, a chambermaid.

He joined the Union of Communist Youth in 1944 and the Communist Party in 1953 and made a career in the Communist nomenklatura, becoming a secretary of the Central Committee of the Union of Communist Youth in 1956 and a member of the Central Committee of the Romanian Communist Party in 1965. At one point, he served as the head of the Central Committee's Department of Propaganda. Iliescu later served as Minister for Youth-related Issues between 1967 and 1971.

Iliescu married Nina Șerbănescu in 1951; they have no children, not by choice but because they could not, as Nina had three miscarriages. Born in Oltenița, Iliescu studied fluid mechanics at the Bucharest Polytechnic Institute and then as a foreign student at the Energy Institute of the Moscow University. During his stay in Moscow, he was the secretary of the "Association of Romanian Students" it is alleged that he met Mikhail Gorbachev, although Iliescu always denied this. However, years later, President Nicolae Ceaușescu probably believed that there was a connection between the two, since during Gorbachev's visit to Romania in July 1989, Iliescu was sent outside of Bucharest to prevent any contact.

He joined the Communist Party in 1953 and became a member of its Central Committee in 1965, however beginning with 1971 he was gradually marginalized by Nicolae Ceaușescu. He had a leading role in the Romanian Revolution, becoming the country's President in December 1989. In May 1990, he became Romania's first freely elected head of state. After a new constitution was approved by popular referendum, he served a further two terms as President, from 1992 to 1996, and from 2000 to 2004, separated by the presidency of Emil Constantinescu, who defeated him in 1996.

However, in 1971, Ceaușescu felt threatened by Iliescu, who was seen as Ceaușescu's heir apparent. He was marginalized and removed from all major political offices, becoming vice-president of the Timiș County Council (1971–1974), and later President of the Iași Council (1974–1979). Until 1989, he was in charge of the Editura Tehnică publishing house. For most of the 1980s (if not before), he was tailed by the Securitate (secret police), as he was known to oppose Ceaușescu's harsh rule.

Though enjoying a certain popularity due to his opposition to Ceaușescu and image as a revolutionary, his political career after 1989 was characterized by multiple controversies and scandals. Public opinion regarding his tenure as President is still divided.

He, along with other figures in the leading FSN, was allegedly responsible for calling the Jiu Valley miners to Bucharest on 28 January and 14 June 1990 to end the protests of the citizens gathered in University Square, Bucharest, protests aimed against the ex-communist Leaders of Romania (like himself). The pejorative term for this demonstration was the Golaniad (from the Romanian golan, rascal). On 13 June, an attempt of the authorities to remove from the square around 100 protesters, which had remained in the street even after the May elections had confirmed Iliescu and the FSN, resulted in attacks against several state institutions, such as the Ministry of Interior, the Bucharest Police Headquarters and the National Television. Iliescu issued a call to the Romanian people to come and defend the government, prompting several group of miners to descend on the capital, armed with wooden clubs and bats. They trashed the University of Bucharest, some newspaper offices and the headquarters of opposition parties, claiming that they were havens of decadence and immorality - drugs, firearms and munitions, "an automatic typewriter", and fake currency. The June 1990 Mineriad in particular was widely criticized both at home and internationally, with one Historian (Andrei Pippidi) comparing the events to Nazi Germany's Kristallnacht. Government inquiries later established that the miners were infiltrated and instigated by former Securitate operatives. In February 1994 a Bucharest court "found two security officers, Colonel Ion. Nicolae and warrant officer Corneliu Dumitrescu, guilty of ransacking the house of Ion Rațiu, a leading figure in the National Peasant Christian Democratic Party, during the miners’ incursion, and stealing $100,000."

The new Constitution was adopted in 1991, and in 1992 he won a second term when he received 61% of the vote. He immediately suspended his NSDF membership; the Constitution does not allow the President to be a formal member of a political party during his term. After the June 16, 1991 bridge of flowers, Mircea Snegur, then President of the Republic of Moldova, proposed in August Iliescu the union of the two countries. Snegur would become the Vice President of Romania. However, his proposal was ignored. He ran for a third time in 1996 but, stripped of media monopoly, he lost to Emil Constantinescu. Over 1,000,000 votes were cancelled, leading to accusations of widespread fraud.

In 1992, three years after the revolution which overthrew the Communist dictatorship, the Romanian government allowed King Michael to return to his country for Easter celebrations, where he drew large crowds. In Bucharest over a million people turned out to see him. Michael's popularity alarmed the government of President Ion Iliescu, so Michael was forbidden to visit Romania again for five years. In 1997, after Iliescu's defeat by Emil Constantinescu, the Romanian Government restored Michael's citizenship and again allowed him to visit the country.

In December 2001, Iliescu pardoned three inmates convicted for bribery, including George Tănase, former Financial Guard head commissioner for Ialomița. Iliescu had to revoke Tănase's pardon a few days later due to the media outcry, claiming that "a legal adviser was superficial in analyzing the case". Later, the humanitarian reasons invoked in the pardon were contradicted by another medical expert opinion. Another controversial pardon was that of Dan Tartagă—a businessman from Brașov that, while drunk, had run over and killed two people on a pedestrian crossing. He was sentenced to three years and a half but was pardoned after only a couple of months. Tartagă was later sentenced to a two-year sentence for fraud.

Ion Iliescu is mentioned in the report of the Council of Europe investigator into illegal activities of the CIA in Europe, Dick Marty. He is pointed out as one of the people who authorized or at least knew about and have to stand accountable for torture prisons at Mihail Kogălniceanu airbase from 2003 to 2005. In April 2015, Iliescu confirmed that he had granted a CIA request for a site in Romania, but was not aware of the nature of the site, describing it as a small gesture of goodwill to an ally in advance of Romania's eventual accession to NATO. Iliescu further stated that had he known of the intended use of the site, he would certainly not have approved the request.

Most controversial of all, on 15 December 2004, a few days before the end of his last term, Iliescu pardoned 47 convicts, including Miron Cozma, the leader of the miners during the early 1990s, who had been sentenced in 1999 to 18 years in prison in conjunction with the September 1991 Mineriad. This has attracted harsh criticism from all Romanian media. Many of the pardoned had been convicted for corruption or other economic crimes, while one had been imprisoned for his involvement in the attempts at suppressing the 1989 Revolution.

In the PSD elections of 21 April 2005, Iliescu lost the Party presidency to Mircea Geoană, but was elected as honorary President of the party in 2006, a position without official executive authority in the party.

In the last days of his President mandate, he awarded the National Order Steaua României (rank of ceremonial knighthood) to the ultra-nationalist controversial Politician Corneliu Vadim Tudor, a gesture which drew criticism in the press and prompted Nobel Peace Prize winner Elie Wiesel, fifteen Radio Free Europe journalists, Timișoara mayor Gheorghe Ciuhandu, Songwriter Alexandru Andrieș, and Historian Randolph Braham to return their Romanian honours in protest. The leader of the Democratic Union of Hungarians in Romania, Béla Markó, did not show up to claim the award he received on the same occasion. President Traian Băsescu revoked the award granted to Tudor on 24 May 2007, but a lawsuit is ongoing even after Băsescu's decree was declared constitutional.