Tsutomu Yamaguchi, the only person officially recognised as a survivor of the two attacks, succumbed to stomach cancer on Monday.
"I thanked my father for leaving us with the treasure that was his effort to call for world peace," said his daughter Toshiko Yamasaki, 61. He is survived by a son, two daughters and five grandchildren.
"It is to our regret that we have lost a valuable story teller," said Nagasaki mayor Tomihisa Taue.
"His painful experience of being bombed twice in Hiroshima and Nagasaki drew worldwide attention."
Severe burns to arms
Yamaguchi, then an engineer at the Mitsubishi Heavy Industries shipyard in Nagasaki, was exposed to the first atomic blast in Hiroshima on August 6, 1945, when he was there for a work assignment.
He was on a street about two kilometres from ground zero when the bomb detonated.
With severe burns to his arms, he returned to Nagasaki two days later to join his family.
Yamaguchi was exposed to the second atomic explosion the next day when he was reporting about the Hiroshima holocaust at his work place, about three kilometers from the epicentre.
"I thought the mushroom cloud had followed me there," he said later.
The atomic blasts killed an estimated 140,000 people in Hiroshima and 74,000 others in Nagaski, leaving numerous others with ailments linked to radioactive irradiation.
Yamaguchi started to publicly talk about his atomic-bomb experience only in 2005 when he lost his second son - who survived the Nagaski bombing as an infant - to cancer.
'My mission is over'
In 2006, he was featured in a documentary film, entitled "Niju Hibaku (double irradiation)" with seven others who were known to have survived the two attacks.
The documentary was screened at the United Nations headquarters in New York the same year, featuring Yamaguchi as a guest speaker.
He became the only person officially recognised as a double A-bomb survivor last year when the city of Nagasaki acknowledged he was also bombed in Hiroshima.
Yamaguchi was diagnosed with stomach cancer in 2006 and was hospitalised last August.
"I think this will be my last lecture," Yamaguchi told a seminar in Nagasaki last June. I hope the baton will be passed to other people."
On December 22, US director James Cameron of Titanic and Avatar fame called on him to outline his idea of shooting a film on atomic bombs, his daughter said.
"My father had eagerly waited for the director to come. He seemed to gather strength after the meeting," Yamasaki said.
"He was heard saying, 'My mission is over.'"
