Agnew, who had chronic lymphocytic leukemia, died Sunday at his home in Solana Beach, Calif., his family said in a statement released Tuesday by the Los Alamos National Laboratory.
Agnew played a role in significant events of the nuclear age, helping build the world's first reactor, flying alongside the Enola Gay when it dropped its devastating load on Hiroshima and heading Los Alamos' weaponry division in postwar period and guiding the development of many new weapons.
"Harold was an innovator," Los Alamos National Laboratory historian Alan Carr told the Los Angeles Times. "The vast majority of weapons in the nuclear stockpile were designed at Los Alamos and Harold had a hand in designing most of them -- I'd say about 75 percent."
After World War II, when many of the leading scientists of the early nuclear era, including Albert Einstein and J. Robert Oppenheimer, expressed moral reservation about the use of the bomb and its consequences, Agnew remained unapologetic.
"My feeling toward Hiroshima and the Japanese was, they bloody well deserved it," he told the Times in 1984. "The whole damn thing has been turned around as if we were the bad guys."
Agnew was born in Denver March 28, 1921, earned a degree in chemistry from the University of Denver in 1942 and joined a research group headed by Enrico Fermi, the Italian-born physicist whose work led to the first controlled nuclear chain reaction in Chicago Dec. 2, 1942.
He followed Fermi to the Manhattan Project to help develop the atomic bomb.
In 1979, Agnew left Los Alamos to become president of General Atomics, guiding work on new types of nuclear reactors until his retirement in 1985.