After he finished high school, Fernandez returned to his country and started law studies in the Universidad Autonoma de Santo Domingo. He joined the Dominican Liberation Party when it was founded in 1973 by former Dominican President Juan Bosch, leaving the Dominican Revolutionary Party (PRD) to do so. Fernández was known as a close pupil of Bosch. He accompanied him as a vice-presidential candidate in the 1994 presidential election.

Fernández married for the first time to Rocío Domínguez Quezada in 1987 (the daughter of Juan Domínguez and the mayoress of Jarabacoa Josefa Piedad Quezada), and had two children, Omar and Nicole. They divorced in 1996. In 2003, Fernández married Margarita Cedeño Lizardo, his former Presidential Legal Adviser and an associate attorney of his law firm, having another child, Yolanda América María.

The 1994 presidential election results, which resulted in a reelection of President Joaquín Balaguer, were widely opposed by runner-up opposition leader José Francisco Peña Gómez and his party. In the meantime, the international community rose their concerns on the tightening of the political tensions and fear the country was on the verge of a civil uprising. Upon an official agreement widely known as “Pact for Democracy” (“Pacto por la Democracia” in Spanish) a special election was scheduled for May 1996 and President Balaguer acceded to not run in this election as part of the arranged treaty. PRSC opted to present Vice-President Jacinto Peynado as their candidate, however PRSC supporters feared he wasn’t properly endorsed by their historical leader, therefore creating a vast independent mass of electors.

In the first-round election on 16 May 1996 Fernández received 38.9% of the votes in the first round achieving second place behind José Francisco Peña Gómez and PRD. Peynado obtained 15% of the votes, the lowest outcome in Balaguer’s party history which initiated a long-term downfall. After 2010 Presidential election, PRSC lost briefly its official recognition as a majority party, but regained it after an alliance with PLD was announced by Leonel Fernández for 2010 mid-term elections, in where both parties achieved 100% of Senate positions and obtained over 2/3 of the seats on the Chamber of Deputies. In the run-off election, held on 30 June 1996, the “Patriotic Front” integrated, and Fernández thus secured 51.2% of the votes to win the election. He was sworn in as President on 16 August 1996, succeeding Balaguer.

Leonel Antonio Fernandez graduated with honors in law from the Autonomous University of Santo Domingo. For a while he worked as a teacher and a journalist before getting into politics. His interest in politics derived from his outstanding student leader abilities, and his dominant role in the General Secretariat of the Students’ Association University of Santo Domingo. Moreover, Leonel Antonio Fernandez partook in many protests against the Balaguer regime, was a member of the Central and Political Committees of the Party of Dominican Liberation, which was founded by Juan Bosch. He began expanding his political interests and won Presidency in the Dominican Republic in 1996.

In 2000, Fernández was unable to run for a consecutive term as the 1994 constitution prohibited it. His party held primaries to elect a presidential nominee, which were won by Danilo Medina with 52% of the votes against Vice-President Jaime David Fernández Mirabal. Medina, who was sworn as the PLD candidate for the 2000 presidential election, was defeated by the PRD candidate, Hipólito Mejía. Mejia did not satisfy the 50% rule to avoid a feared run-off election, but Medina, who placed second, recognized Mejia’s win immediately.

In 2000, Leonel Fernandez founded the Fundación Global Democracia y Desarrollo (FUNGLODE), headquartered in Santo Domingo, as a think tank as well as a resource and training center. In 2002, he founded a sister organisation, the Global Foundation for Democracy and Development, dedicated to promoting collaboration between organizations in the United States and the Dominican Republic.

Fernández was elected to a second term of office in the presidential election held on 16 May 2004 with an absolute majority and the second highest percentage ever in Dominican history of 57%. He was sworn in on 16 August 2004. His victory was due in large part to the collapse of the Dominican economy. This collapse has been attributed to skyrocketing oil prices and a slumping international economy post-9/11, excessive borrowing under the Mejía administration and the failure of three banks in the country: Banco Intercontinental (Baninter), Bancrédito and Banco Mercantil. In an effort to avoid further economic chaos, the Mejía administration effectively underwrote all three banks, repaying their customers but generating even greater public debt. Some of the bankers involved have been or are on trial, yet the extent of their fraud is hard to measure as their widespread largess with officials within both the Fernández and Mejía administrations has made it nearly impossible to achieve a true accounting of what conditions led to the bank’s demise.

During the Mejía administration, the constitution was changed to again allow the immediate re-election of presidents. Although Mejía was ultimately unsuccessful in his bid for re-election, this opened the door for Fernández to attempt a similar run. On 7 May 2007, despite great controversy over whether the PLD party should support the concept of re-election, Fernández won the PLD primary election with a 72% voting percentage against his former Chief of Staff and confidant, Danilo Medina.

Fernández received World Peace Culture Award on 6 April 2008.

Fernández was declared the victor of the 2008 election, defeating six others, including the PRD candidate and former president Mejía’s right-hand man, Miguel Vargas, and the PRSC’s candidate, Amable Aristy. Fernández defeated Vargas 53% to 41%. Vargas accepted the defeat the same day, 16 May. Fernández was sworn in for his third term on 16 August 2008, with the traditional ceremony at the National Congress. An ongoing international economic which occurred during this time did not prevent the Dominican Republic from remaining the fastest growing economy in Latin America, with a growth rate of 3.5 percent in 2009 and a projected growth rate of 6.5 to 7 percent in 2010.

On 20 October 2018, the 42nd Plenary Assembly of the World Federation of United Nations Associations (WFUNA) unanimously elected him president of the global body for a 3-year renewable term thus succeeding to South Korea’s senior Ambassador Park Soo-gil and former International Atomic Energy Agency Director General, Hans Blix.