Born Toshio Okumura (奥村 利夫 Okumura Toshio) on 29 November 1931. He was the son of kabuki performer Katsutoji Kineya (Kineya Katsutōji) who was renowned for his nagauta and shamisen skills, and younger brother of actor Tomisaburo Wakayama (Wakayama Tomisaburō).

Shintaro Katsu began his career in entertainment as a shamisen player. He switched to acting because he noticed it was better paid. In the 1960s he starred simultaneously in three long-running series of films, the Akumyo series, the Hoodlum Soldier series, and the Zatoichi series.

He was the husband of Actress Tamao Nakamura (married in 1962), and father of actor Ryutaro Gan (Gan Ryūtarō).

In her book, Geisha, A Life, Kyoto geisha Mineko Iwasaki claimed to have had a long time affair with Katsu, whom she calls by his given name, Toshio. The affair ended in 1976, and eventually the two became good friends until his death.

Katsu had a troubled personal life. A heavy drinker, Katsu had several skirmishes with the law regarding drug use as well, including marijuana, opium and cocaine with arrests in 1978, 1990 and 1992.

He had also developed a reputation as a troublemaker on set. When Director Akira Kurosawa cast him for the lead role in Kagemusha (1980), Katsu left before the first day of shooting was over. Though accounts differ as to the incident, the most consistent one details Katsu's clash with Kurosawa regarding bringing his own film crew to the set (to film Kurosawa in action for later exhibition to his own acting students). Kurosawa is reputed to have taken great offense at this, resulting in Katsu's termination (he was replaced by Tatsuya Nakadai). In her recent book, Waiting on the Weather, about her experiences with Director Kurosawa, script supervisor Teruyo Nogami chalks the differences between Katsu and Kurosawa up to a personality clash that had unfortunate artistic results.