Lázaro Cárdenas del Río (Spanish pronunciation: [ˈlasaɾo ˈkaɾðenas] ^ⓘ; 21 May 1895 – 19 October 1970) was a Mexican army officer and politician who served as president of Mexico from 1934 to 1940. Previously, he served as a general in the Constitutional Army during the Mexican Revolution and as Governor of Michoacán and President of the Institutional Revolutionary Party. He later served as the Secretary of National Defence. During his presidency, which is considered the end of the Maximato, he implemented massive land reform programs, led the expropriation of the country's oil industry, and implemented many key social reforms.

Born in Jiquilpan, Michoacán, to a working-class family, Cárdenas joined the Mexican Revolution and became a general in the Constitutionalist Army. Although he was not from the state of Sonora, whose revolutionary generals dominated Mexican politics in the 1920s, Cárdenas was hand-picked by Plutarco Elías Calles, Sonoran general and former president of Mexico, as a presidential candidate and won in the 1934 general election.

After founding the National Revolutionary Party (PNR) in the wake of the assassination of president-elect Álvaro Obregón, Plutarco Elías Calles had unofficially remained in power during the Maximato (1928–1934) and expected to maintain that role when Cárdenas took office. Cárdenas, however, out-maneuvered him politically and forced Calles into exile. He established the structure of the National Revolutionary Party, eventually renamed the Party of the Mexican Revolution (PRM), on the sectoral representation of peasant leagues, labor union confederations, and the Mexican Army. Cárdenas's incorporation of the army into the party structure was a deliberate move to diminish the power of the military and prevent their intervention in politics through coups d'état.

A left-wing economic nationalist, Cárdenas led the expropriation of the Mexican oil industry and the creation of the state-owned oil company Pemex in 1938. He implemented large-scale land reform programs in Mexico, redistributing large estates to smallholders in lands termed ejidos. He created the National Polytechnic Institute (IPN) and El Colegio de México (Colmex). His foreign policy supported and gave asylum to Republicans during the Spanish Civil War. An achievement of Cárdenas was his complete surrender of power in December 1940 to his successor, Manuel Ávila Camacho, who was a political moderate without a distinguished military record.

Cárdenas has been praised as "the greatest constructive radical of the Mexican Revolution", for implementing its ideals, but has also been criticized as an "authoritarian populist". He was the first Mexican president to serve for a sexenio, a practice that continues today. According to numerous opinion polls and analysts, Cárdenas is the most popular Mexican president of the 20th century.

Lázaro Cárdenas del Río was born on 21 May 1895, one of eight children in a lower-middle-class family in the village of Jiquilpan, Michoacán, where his father (Dámaso Cárdenas Pineda) owned a billiard hall. His family was of prominent Purépecha ancestry. After the death of his father in 1911, from the age of 16, Cárdenas supported his family (including his mother and seven younger siblings). By the time he reached 18, he had worked as a tax collector, a printer's devil, and a jail keeper. Although he left school when he was eleven, he used every opportunity to educate himself and read widely throughout his life, especially works of history.

Cárdenas set his sights on becoming a teacher but was drawn into the military during the Mexican Revolution after Victoriano Huerta overthrew President Francisco Madero in February 1913, although Michoacán was far from the revolutionary action that had brought Madero to the Mexican presidency. After Huerta's coup and Madero's assassination, Cárdenas joined a group of Zapatistas, but Huerta's forces scattered the group, where Cárdenas had served as captain and paymaster. Given that revolutionary forces were voluntary organizations, his position of leadership points to his skills and his being a paymaster to the perception that he would be honest in financial matters. Both characteristics followed him through his subsequent career. He escaped the Federal forces in Michoacán and moved north, where he served initially with Álvaro Obregón, then Pancho Villa, and after 1915 when Villa was defeated by Obregón to Plutarco Elías Calles, who served Constitutionalist leader Venustiano Carranza. Although Cárdenas was from the southern state of Michoacán, his key experiences in the Revolution were with Constitutionalist northerners, whose faction won. In particular, he served under Calles, who tasked him with military operations against Yaqui Indians and against Zapatistas in Michoacán and Jalisco, during which time he rose to a field command as general. In 1920, after Carranza was overthrown by northern generals, Cárdenas was given the rank of brigadier general at the age of 25. Cárdenas was appointed provisional governor of his home state of Michoacán under the brief presidency of Adolfo de la Huerta.

Cárdenas was a political protégé of Calles, but his ideological mentor was revolutionary General Francisco J. Múgica, a strongly anticlerical, secular socialist. President Calles appointed Cárdenas Chief of Military Operations in the Huasteca, an oil-producing region on the Gulf Coast. Cárdenas saw first-hand the operations of foreign oil companies. In the Huasteca, U.S. oil companies extracted oil, avoided taxes owed to the Mexican government, and treated the region as “conquered territory.” Múgica also was posted to the Huasteca and he and Cárdenas became close. During their time in the Huasteca, Múgica told Cárdenas that “socialism [is] the appropriate doctrine for resolving conflicts in Mexico.”