Was educated in local schools in Ponce, in Valley Forge Military Academy in Pennsylvania, and at Johns Hopkins University. He finally received a law degree from the University of Puerto Rico.
After privately practicing law in 1959-1960, Hernández served as commissioner of public service (1960-62). Between 1961 and 1966 he was professor of law in the Catholic University of Puerto Rico in Ponce.
Hernández Colón entered politics as secretary of justice, in which capacity he was responsible for writing the Political Code, Mortgage Code, and Plebiscite Act of 1967. In 1968 he was elected senator-at-large, and between 1969 and 1972 he was president of the Popular Democratic-controlled Senate. In 1969 he was chosen president of the party.
Hernández Colón served as governor of Puerto Rico from 1973 to 1977 but, unable to deal adequately with the island’s growing economic problems, was defeated for reelection. He remained head of the Popular Democratic Party and was its victorious candidate for governor in 1984.