Diosdado Cabello was born in El Furrial, Monagas State. His background is in engineering. He has an undergraduate degree in systems engineering from the Instituto Universitario Politécnico de las Fuerzas Armadas Nacionales and a graduate degree in engineering project management from the Andrés Bello Catholic University.
While at Instituto Universitario Politécnico de las Fuerzas Armadas Nacionales, Cabello befriended Hugo Chávez and they played on the same baseball team.
During Chávez’s abortive coup d'état of February 1992, Cabello led a group of four tanks to attack Miraflores Palace. Cabello was jailed for his participation in the coup, though President Rafael Caldera, who had prior knowledge of the coup, later pardoned him with the rest of the coup participants and Cabello was released after only two years without any charges.
Following Chávez’s 1998 electoral victory, he helped set up the pro-Chávez grassroots civil society organizations known as "Bolivarian Circles" which have been compared to Cuba's Committees for the Defence of the Revolution and are parent organizations for the Colectivos. He was head of telecoms regulator Conatel during the time the market was opened to competition. In May 2001, he became Chavez' chief of staff, and was appointed Vice President by President Hugo Chávez on 13 January 2002, replacing Adina Bastidas. As such, he was responsible to both the president and the National Assembly, and for the relations between the executive and legislative branches of the government.
On 13 April 2002, he took on the duties of the presidency on a temporary basis, replacing Pedro Carmona, head of the Venezuelan Chamber of Commerce, as interim president during the coup d'état attempt when Chávez was kept prisoner and was consequently absent from office. Upon taking office, Cabello said that "I, Diosdado Cabello, am assuming the presidency until such time as the president of the republic, Hugo Chávez Frías, appears." A few hours later, Chávez was back in office. This made Cabello’s presidency the world’s second briefest, after that of Mexican President Pedro Lascuráin.
On 28 April 2002, Cabello was replaced as Vice President by José Vicente Rangel. Cabello was named interior minister in May 2002, and then infrastructure minister in January 2003.
In October 2004, Cabello was elected to a four-year term as Governor of Miranda State. He lost the 2008 election to Henrique Capriles Radonski, and was subsequently appointed Public Works & Housing Minister. In November 2009 he was additionally appointed head of Conatel.
On 11 December 2011, Cabello was installed as the Vice-President of the United Socialist Party (PSUV), thus becoming the second most powerful figure in the party after Hugo Chávez.
Cabello was appointed president of the National Assembly in early 2012 and was re-elected to that post in January 2013.
Cabello’s status after the death of Hugo Chávez was disputed. Some argue that Cabello was constitutionally required to be the acting President, but Nicolás Maduro held the position.
Cabello has his own weekly program on state TV, Con el Mazo Dando (Hitting with the Sledge Hammer). In that program, Cabello talks about the government's view on many political issues and presents accusations against the opposition. The Inter-American Commission on Human Rights (IACHR) has expressed concerns about how the program has intimidated people that went to the IACHR denouncing the government. Some Venezuelan commentators have compared the use of illegally recorded private conversations on programs such as Cabello's to the practices in place in the German Democratic Republic as shown in the film The Life of Others.
Amnesty International has denounced the way in which Cabello has revealed details on the travel arrangements of two human rights defenders in his program and how he routinely shows state monitoring of people that may disagree with the government.
Cabello was nicknamed "the octopus" for having "tentacles everywhere". He is very influential in the Venezuelan government, using a network of patronage throughout the military, ministries and pro-government militias.
Information presented to the United States State Department by Stratfor claimed that Cabello was "head of one of the major centers of corruption in Venezuela." A Wikileaked U.S. Embassy cable from 2009 characterized Cabello as a “major pole” of corruption within the regime, describing him as “amassing great power and control over the regime’s apparatus as well as a private fortune, often through intimidation behind the scenes.” The communiqué likewise created speculation that “Chavez himself might be concerned about Cabello's growing influence but unable to diminish it.” He is described by a contributor to The Atlantic as the "Frank Underwood" of Venezuela under whose watch the National Assembly of Venezuela has made a habit of ignoring constitutional hurdles entirely—at various times preventing opposition members from speaking in session, suspending their salaries, stripping particularly problematic legislators of parliamentary immunity, and, on one occasion, even presiding over the physical beating of unfriendly lawmakers while the assembly was meeting.
Cabello has been accused on several occasions of nepotism. His wife, Marlenys Contreras, served as a member of the National Assembly until she became minister of tourism in 2015. Cabello’s sister, Glenna, is a political scientist and current Counsellor of the Venezuelan Permanent Mission to the United Nations. His brother, José David, previously minister of infrastructure, was later in charge of the nation’s taxes as head of SENIAT, Venezuela’s revenue service. Now José David is minister of Industries.
Allegations of corruption involving Cabello includes being head of an international drug trafficking organization, accepting bribes from Derwick Associates for public works projects in Venezuela, using nepotism to reward friends and family members and directing colectivos while paying them with funds from Petróleos de Venezuela. In 2013, there were at least 17 formal corruption allegations lodged against Cabello in Venezuela's prosecutors office.
On 27 January 2015, reports accusing Cabello of drug trafficking emerged. In a series of investigations by the United States government, it was stated that Cabello's alleged involvement in the drug trade as the "capo" [sic] (head) of the Cartel of the Suns (Spanish Cartél de los soles), had also involved high-ranking generals of Venezuelan military. Cabello has also been accused by the Human Rights Foundation president of corruption and drug trafficking.
In mid-July 2017, reporters in Washington, D.C. observed an increased security presence surrounding United States Senator Marco Rubio. A month later on 13 August 2017, The Miami Herald reported that Diosdado Cabello had initiated an assassination plot targeting Rubio, allegedly contacting Mexican nationals to discuss killing Rubio. Rubio, who is a critic of the Venezuelan government, has led an effort in the United States government to take action against corrupt officials of the Latin American government, often singling out Cabello. The Department of Homeland Security could not verify all of the details involved in the threat, though the plan was serious enough that multiple law enforcement agencies were contacted about the incident and Rubio's security detail had increased in size.