Born in Tokyo, Iwaki studied in the instrumental music department of the Tokyo University of Fine Arts. He made his conducting debut in 1956 with the NHK Symphony, and was appointed conductor in 1963. He gave his first concerts with the MSO in 1973 and was its chief conductor from 1974 to 1989. The MSO appointed him its first conductor laureate in 1990.
In Europe Iwaki conducted the Berlin and Vienna Philharmonics, the Royal Concertgebouw Orchestra, and Leipzig's Gewandhaus Orchestra. He also led the MSO on its first tour to Japan in 1987.
He was awarded numerous prizes and honors; in 1990, the French government named him an Officer of the Order of Arts and Letters; he was awarded Honorary Membership of the Order of Australia in 1985.
Iwaki's enthusiasm for contemporary music earned him the nickname "premiere maniac," according to the MSO, which said on its web site that in one three-year period he gave 236 performances of Japanese and contemporary works, including many world premieres.
According to the Associated Press, Iwaki also founded Japan's only permanent professional chamber orchestra, Orchestra Ensemble Kanazawa.
The MSO quotes principal oboe Jeffrey Crellin as saying, "It seems to me that the orchestra's playing is always much tighter with Iwaki conducting. The orchestra and the audience respond to the extra electricity he generates."
On New Year's Eve 2004/05, at age 72, Iwaki conducted members of the NHK Symphony and other Tokyo ensembles in a marathon performance of all nine of Beethoven's symphonies. "Beethoven is special," he told reporters. "I'd be happy even if I died during the performance."