Hiroshi Koizumi was born on August 12, 1926, in Kamakura City, Japan. He graduated from Keio University in 1948. He joined NHK television as an announcer and was selected to be part of the third-annual Tôhô New Face acting class in 1951. The following year, Koizumi made his acting debut in Tôhô Studios' Minato e kita otoko (1952). When the studios' sci-fi films took flight, Koizumi became a regular in those films, first starring as pilot hero Shoichi Tsukioka in Godzilla's first sequel, Şehirleri Mahveden Canavar (1955). After appearing in the film, he became most notable for portraying doctors, professors, or scientists.

It is interesting to note that Koizumi appeared in Godzilla's 10th-, 20th-, and 30th-anniversary films: Godzilla Beklenmeyen Canavar (1964), Gidorah Canavarlar Canavarı (1964), Canavarlar Çarpışıyor (1974) and Godzilla'nın Dönüşü (1985), starring as a professor or a scientist. Koizumi is one of the few actors to appear in more than one Godzilla movie as the same character. He played Professor Miura in Godzilla Beklenmeyen Canavar (1964) and Gidorah Canavarlar Canavarı (1964) (or Professor Murai). In the 1990s, Koizumi worked on a few films as producer and also served as host of Japan's long-running game show "Quiz Grand Prix."