Leslie was a regular on NBC's The Richard Boone Show, which garnered her an Emmy Award nomination for Outstanding Single Performance by an Actress in a Leading Role for her work in the episode "Statement of Fact." Media critic John Crosby wrote about Leslie's work in that anthology series, "During the season Bethel played everything from a seductive ax murderess to a dumb gangster's moll, to an Irish scrub woman, through a whole series of witchy mothers."
A poll of media critics and editors named her Most Promising New Talent in Radio Television Daily's 1963 All-American Favorites—Television.
Leslie's 1986 Broadway portrayal of a drug-addicted mother in Long Day's Journey into Night brought her a Tony Award for Best Featured Actress.
Leslie began working in television in the 1940s and frequently was a guest on the many anthology series popular in the early to mid-1950s, such as Studio One and Playhouse 90. She appeared with Ronald W. Reagan and Stafford Repp in the 1960 episode "The Way Home" of CBS's The DuPont Show with June Allyson.
While a 13-year-old student at Brearley School, Leslie was discovered by George Abbott, who cast her in the play Snafu in 1944. In a 1965 newspaper article, Leslie described herself as "a 'quick study' -- able to learn my lines rather fast."
Over the next four decades she appeared in a number of Broadway productions, including Goodbye, My Fancy (1948), The Time of the Cuckoo (1952), Inherit the Wind (1955), Catch Me If You Can (1965), and Long Day's Journey Into Night (1986).
In 1950, Leslie was cast as Cornelia Otis Skinner in The Girls, a television series based on the author's Our Hearts Were Young and Gay. She departed the show after two months to appear with Helen Hayes in the play The Wisteria Trees, adapted from Anton Chekhov's The Cherry Orchard by Joshua Logan.
In 1962, she portrayed the part of Martha Hastings in the episode, "The Long Count", on CBS's Rawhide.
A poll of media critics and editors named her Most Promising New Talent in Radio Television Daily's 1963 All-American Favorites—Television.
Leslie's debut in feature films came in 1964 in Captain Newman, M.D.. Her feature film credits include A Rage to Live (1965), The Molly Maguires (1970), with Sean Connery, Dr. Cook's Garden (1971), Old Boyfriends (1979), Ironweed (1987), Message in a Bottle (1999) and Uninvited (1999).
Leslie's 1986 Broadway portrayal of a drug-addicted mother in Long Day's Journey into Night brought her a Tony Award for Best Featured Actress.
