Zapatero was the son of a lawyer and the grandson of a Republican army officer executed by Gen. Francisco Franco (Franco, Francisco)'s forces during the Spanish Civil War. He attended the University of León and became a member of the university's law faculty after graduating in 1982. In 1986 Zapatero, who had joined the Spanish Socialist Workers' Party (Partido Socialista Obrero Español; PSOE) in 1979, was elected to the national parliament, becoming its youngest member. Two years later he was made general secretary of the PSOE's León provincial federation. Zapatero established a reputation as a capable, hardworking deputy, but he held no public office in the socialist administrations that governed Spain between 1982 and 1996. In July 2000, however, he defeated three other candidates to become the party's general secretary. He promised to modernize both the PSOE and its policies, offering an agenda that revolved around economic efficiency, women's rights, democratic participation, laicism, and “constructive” opposition to the conservative Popular Party (PP) government.

As the 2004 general elections neared, opinion polls suggested an easy win for the PP. On March 11, 2004, however, Madrid suffered a series of terrorist (terrorism) attacks, and Prime Minister José María Aznar (Aznar, José María) and his PP government drew criticism for their attempts to blame the Basque separatist group ETA even after members of the Islamist militant group al-Qaeda (Qaeda, al-) were arrested. Aided by the voter backlash, the PSOE won a surprise victory at the elections on March 14. Zapatero was sworn in as prime minister on April 17, 2004, and he subsequently appointed a cabinet that combined established and emerging figures, half of whom were women. Within weeks of taking office, he also followed through on a campaign pledge to withdraw troops serving in Iraq. (See Iraq War.) Zapatero's government supported a number of social reforms, including the legalization of same-sex marriage and the criminalization of domestic violence. In response to two long-standing issues, the status of Catalonia and of the Basque Country, Zapatero supported the declaration of nationhood for Catalonia in 2006 and pledged not to yield to ETA terrorism, respectively.

The PSOE triumphed again in the 2008 general elections after a fiercely battled campaign, though it failed to gain an absolute majority. Zapatero pledged to boost Spain's slumping economy and to continue his agenda of social and political reform.