Born Isamu Yamada (山田勇 Yamada Isamu) in Asahikawa (Hokkaido), he adopted his current stage name while directing the Manga Trio manzai troupe from 1959 to 1968.

He became governor of Osaka prefecture in 1995, running as an independent and joining the Liberal Democratic Party (LDP) after his election.

He enjoyed great popularity as governor, mostly due to his existing fame as a comedian.

Yokoyama denied the charges, but the Osaka District Court found him liable for ¥11 million in damages, following a highly publicized trial in which the plaintiff testified from behind an opaque screen to avoid revealing her identity.

Following the judgment, Yokoyama resigned: he was replaced by a female LDP bureaucrat, Fusae Ohta.