- Helen Keller (Patty Duke): A young, blind and deaf girl who becomes the focus of the film's story. She is unable to communicate with the outside world and is frustrated by her inability to understand what is going on around her.
- Annie Sullivan (Anne Bancroft): A dedicated and experienced teacher who is hired to work with Helen Keller. She is determined to help Helen overcome her disabilities and learn how to communicate with the world.
- Captain Arthur Keller (Victor Jory): Helen's father, who is a former Confederate Army officer. He is a loving and supportive father, but is also strict and has high expectations for his daughter.
- Kate Keller (Inga Swenson): Helen's mother, who is a kind and gentle woman. She is devoted to her daughter and wants to help her in any way she can.
- James Keller (William Gibson): Helen's older brother, who is a loving and protective sibling. He is also supportive of Annie's work with Helen.
Other characters:
- Aunt Ev (Kathleen Comegys): Helen's aunt, who is a kind and caring woman. She helps Annie to take care of Helen and is a source of support for the family.
- Mr. Anagnos (Andrew Prine): The director of the Perkins School for the Blind, where Helen Keller is sent to study. He is a kind and understanding man who believes in Helen's potential.
- Miss Kinnian (Melissa Gilbert): A young teacher at the Perkins School for the Blind who helps Helen to learn how to sign. She is a kind and patient woman who is dedicated to her work with Helen.
- John Macy (Jack Soo): A reporter for the Boston Herald who writes about Helen Keller's story. He is a kind and compassionate man who is inspired by Helen's courage and determination.
- Thomas Edison (Burgess Meredith): A famous inventor who meets Helen Keller and is inspired by her story. He becomes a friend and mentor to Helen and helps her to achieve her dreams.