Although Patty Duke had been playing Helen Keller in the play for more than year, she almost didn't get the part in the film adaptation. The studio felt that being a teenager, she looked too old to play a seven-year-old. However, they decided to use Duke after deciding to use Anne Bancroft, who played Duke's original Annie Sullivan in the play.
The original Broadway production of "The Miracle Worker" opened at the Playhouse Theater on October 19, 1959, ran for 719 performances, and won the 1960 Tony Award for Best Play. Anne Bancroft (winner of the 1960 Tony Award for Best Actress in a Play) and Patty Duke recreated their stage roles in the movie. Also in the opening night cast were Kathleen Comegys as Aunt Ev and Beah Richards as Viney, both originating their movie roles. William Gibson wrote the teleplay, the stage play and the screenplay.
Several actors in studio records/casting call lists did not appear or were not identifiable in this movie. These were (with their character names): Diane Bryan (Martha at Age 5), Donna Bryan (Martha at Age 7), Peggy Burke (II) (Helen at Age 7), Keith Moore (I) (Percy at Age 6), Mindy Sherwood (Helen at Age 5), and Walter Wright Jr. (Percy at Age 8)