The Pittsburgh native began his professional stage career with the Barter Theatre in 1952. Within two years, Weaver was featured in the Off-Broadway production of "The Way of the World" and the following year earned his first Tony Award nomination for his performance in Broadway's "The Chalk Garden". Since the 1950s, he delivered a string of acclaimed stage portrayals ranging from the titular "Miss Lonelyhearts" on Broadway in 1957 to Sherlock Holmes in the 1965 musical "Baker Street" to his Tony-winning role as the headmaster of a Catholic boys' school in "Child's Play" (1970). Weaver subsequently acted in classics ("King Lear" 1991; "The Crucible" 1992), one-man shows ("Lincoln" 1976) and contemporary dramas (Lanford Wilson's "Angels Fall" 1982-83; A.R. Gurney's "The Cocktail Hour" 1990).
Although he has appeared in a modest number of features, Weaver generally delivered strong, memorable performances. His roles have encompassed everything from an Army colonel with a superiority complex in "Fail Safe" (1964), a foundation head who wants to use marine mammals for political purposes in "The Day of the Dolphin" (1973) and the obsessive computer wizard in "The Demon Seed" (1977). Weaver was in the ensemble cast of "Creepshow" (1982) and was the losing political candidate who does not take Richard Gere's advice in Sidney Lumet's "Power" (1986). On the small screen, Weaver began in prestige versions of stage plays and segued to patrician, sometimes vile characters. He was cast as the overbearing father in "The Legend of Lizzie Borden" (NBC, 1975), starring Elizabeth Montgomery, played John Ericson, the designer of the Monitor, a proto-submarine used by the Union forces in the Civil War, in "Ironclads" (TNT, 1991) and portrayed US Senator Everett Dirksen in the 1992 HBO biopic "Citizen Cohn". Over his long career, Weaver has leant his distinctive vocal talents to voice-overs and narrations of films and TV documentaries.