He was the second son of Anastasio Somoza Garcia, who became President of Nicaragua in 1937. Debayle was initially educated in Florida and Long Island before graduating from West Point Military Academy in 1946. The following year, he was appointed head of the National Guard by his father, who had previously given many important posts to family members and close personal friends.
Following his father's assassination in 1956, Debayle elder brother, Luis Somoza Debayle, took over the presidency. In 1967, shortly before the death of his brother, Debayle was himself elected President for the first time. He stood down from the Presidency in 1972, owing to a law which disallowed immediate re-election.
In December of that year, an earthquake hit the nation's capital Managua, killing around ten thousand people, and virtually destroying the city. Martial law was declared, effectively making Debayle, as head of the National Guard, the country's leader once again. It was later discovered that the Somoza family had embezzled much of the international aid which had been sent to help with the aftermath of the earthquake.
Despite this, Debayle was re-elected President in the 1974 election, although the Catholic church had begun to speak against him. By the late 1970s, human rights groups were condemning the record of the Somoza government, and support for the Sandinistas was growing in the country. They eventually overthrew Debayle on July 17, 1979 and he fled to Miami, Florida. Debayle was assisinated by Sandinista agents the following year in Paraguay.