Cariou began his professional career in the chorus of "Damn Yankees" in his native Winnipeg, Canada, and by 1962 was portraying key roles in Shakespeare's "The Taming of the Shrew", "Macbeth" and "The Tempest" as well as other classics at the Stratford Shakespeare Festival in Ontario, Canada. From Shakespeare, Cariou moved on to the Guthrie Theatre in Minneapolis for several seasons, where he again played in the classics. He made it to Broadway in 1968 as Orestes in "The House of Atreus" at the Billy Rose Theatre. During the next two decades, he would be a frequent star on the stages of the Great White Way in such efforts as "Applause", "Night Watch", "A Little Night Music", "Cold Storage", "Sweeney Todd" (for which he received a 1979 Tony Award as Best Actor in a Musical) , "Dance a Little Closer" and "Teddy and Alice".
Cariou made his film debut reprising his role as Frederick Egerman opposite Elizabeth Taylor in the 1977 film adaptation of "A Little Night Music". He has played supporting roles in a handful of films including "The Four Seasons" (1981), written and directed by its star, Alan Alda, "Lady in White" (1988) and "Never Talk to Strangers" (1995). Cariou's one leading role was in "One Man" (1977), a Canadian film in which he played a TV reporter who uncovers a link between pollution and the deaths of children in a poor section of a town.
The majority of Cariou's career has been spent on the stage. Besides returning frequently to the Guthrie and the Stratford Festival, he was artistic director of the Manitoba Theatre Center in the mid-80s, starring in productions of "Equus" and "Cyrano de Bergerac". His TV work has been sporadic, beginning in 1965 with a production of "The Master Builder". In 1985, he made a guest appearance as a charming spy in a "Murder, She Wrote" episode, which reunited him with his "Sweeney Todd" co-star Angela Lansbury. The role proved so popular with the audience that Cariou reprised it on an almost yearly basis during the long run of the CBS series. He won praise for his performance as a father coping with his son's suicide in "Surviving" (ABC, 1985). Cariou also starred in "Love on the Run" (Showtime, 1994) and portrayed Walt Disney in TV biopic "A Dream Is a Wish Your Heart Makes: The Annette Funicello Story" (CBS, 1995).