In 1929, he joined the Peruvian Army as a foot soldier. Due to his excellent discipline, he was selected to attend the Chorrillos Military School in Chorrillos, located near Lima. In 1934 he graduated at the top of his class. He kept close relationships over the years with some of his military colleagues from this era, most of them later on the Centro de Altos Estudios Militares (CAEM).
A dispute with the International Petroleum Company over licenses to the La Brea y Pariñas oil fields in northern Peru sparked a national scandal when a key page of a contract (the 11th) was found missing. This gave the Armed Forces an excuse to seize absolute power and close down Congress. General Velasco seized power on October 3, 1968 in a military coup, deposing the democratically-elected administration of Fernando Belaúnde, under which he served as Commander of the Armed Forces. President Belaúnde was sent into exile, and by October 8 the oil fields in dispute were taken over by the Army.
The coup leaders named their administration the Revolutionary Government of the Armed Forces, with Velasco at its helm as President. Velasco's rule, known as Velascato, was characterized by left-leaning policies, as he nationalised entire industries, expropriated companies in a wide range of activities from fisheries to mining to telecommunications to power production and consolidated them into single industry-centric government-run entities (PescaPeru, MineroPeru, PetroPeru, SiderPeru,ElectroPeru, EnatruPeru, Enafer, Compañia Peruana de Telefonos (CPT), EntelPeru, etc.), and increased government control over economic activity by enforcing those entities as monopolies and preventing any private activity in those sectors. The media were opened for a first time to leftist intellectuals and politicos. A deep education reform was in march looking to include all Peruvians to a new national thinking and feeling; the poor and the most excluded were vindicated and the Día del Indio name was changed to Día del Campesino every June 24th, a traditional holiday of the land, the day of winter equinoccio.
It was also characterised by the increasing use of authoritarian powers, as the administration grew away from tolerating any level of dissent, periodically jailing, deporting and harassing suspected political opponents and repeatedly closing and censoring broadcast and print news media, finally expropriating all of the newspapers in 1974 and sending the publishers into exile.
A cornerstone of his political and economic strategy was the implementation by dictat of an agrarian reform program to expropriate farms and diversify land ownership, much of which had been concentrated in Latifundios owned by a small percentage of the population; opponents of his program claimed that the expropriation was more akin to confiscation, as they were paid in non-tradable bonds that would eventually become worthless by the government's inflationary policies. Need is to say that only less than 2 % of the Peruvian territory is farmable land; 98 % of the territory is weather arid desert with little rain; harsch mountains with very steped terrain; or wild amazon forest.
Ironically, the deposed Belaúnde administration had attempted to implement a milder agrarian reform program, but it had been defeated in Congress by the APRA-UNO coalition with support of the major landowners. Within this framework, the Velasco administration engaged in an aggressive program of import substitution industrialization, imposing tight foreign exchange and trade controls.
In foreign policy, in contrast with his 1970s Latin American contemporaries, which were mostly right-wing military dictatorships, he pursued a partnership with the Soviet bloc, tightening relations with Cuba and Fidel Castro and undertaking major purchases of Soviet military hardware.
Relations between the United States and Peru were tense during Velasco's time in government beginning with the expropriation of the International Petroleum Company (IPC), a subsidiary of Standard Oil, one week after Velasco seized power in 1968. Although the claims over the IPC were ultimately resolved in negotiations between the two governments, disagreements continued over such issues as Peru's claim to 200 mile fishing limit that resulted in the seizure of several US commercial fishing boats and the expropriation of the American copper mining company Cerro de Pasco Corporation. In 1973, Velasco announced during a press conference when questioned about Soviet military advisors in Peru, that the United States Peace Corps was being expelled from Peru.
Economically, the Velasco administration's policies were ultimately unsuccessful. The government ran deeper into debt and was forced to devalue the currency and ran inflationary policies.
Fisheries and agriculture were particularly visible failures; PescaPeru aggressively overfished the anchoveta, a fish used primarily as material for fishmeal production and a key element in the Peruvian sea ecosystem, which resulted in record production for a couple of years but combined with an El Niño event in 1972 led to an absolute collapse that would take over a decade to partially recover, while the badly mismanaged agrarian reform resulted in the creation of thousands of capital-poor and mostly uneducated small farmers whose production and distribution capacity fell substantially short of the pre-reform production. This, coupled with the trade restrictions, led to periodic shortages, rationing, and increased social unrest.
The military commanders of the 1st, 2nd, 3rd, 4th, and 5th military regions declared that Velasco had not achieved most of what the "Peruvian Revolution" had stood for and was unable to continue in his functions. Prime Minister Francisco Morales Bermúdez was then appointed president, by unanimous decision of the new military junta.
Prior to being deposed, Velasco had been seriously ill for at least a year; he had lost a leg to an embolism, and his cognitive abilities and personality were rumoured to have been affected by related circulatory problems. At the time of the coup, he was convalescing in the Presidential winter residence at Chaclacayo, countryside 20 kilometers east of Lima.
General Velasco immediately called for a meeting with his council of ministers, at Government Palace in downtown Lima, where he discovered that there was little or nothing to do.
President Velasco made a last speech to the nation on the evening of August 29, 1975, announcing his decision not to resist the coup because 'Peruvians cannot fight against each other´.
General Velasco kept a low profile in Peruvian politics until his death in 1977.