She retired from film acting when she became an adult, but was active in local theatre productions as a playwright and composer.

[2] In 1931, Knowlden's father got her a screen test for the Paramount Pictures film Women Love Once while the whole family was visiting Hollywood for reasons related to Robert's work.

[6] She was in Greta Garbo and Clark Gable's 1931 film Susan Lenox (Her Fall and Rise) but her scenes were deleted in the final cut.

[11] She was one of three actresses to portray Jessie Pullman (at age eight) in the 1934 Universal Pictures film Imitation of Life.

[14] Knowlden appeared in two Academy Award-nominated films for Best Picture the following year, David Copperfield, and Les Misérables.

David Copperfield was based on Charles Dickens' 1850 novel, directed by George Cukor, and released by MGM.

Les Misérables, directed by Richard Boleslawski, was based on French author Victor Hugo's 1862 novel of the same name.

Having both played the same character in Imitation of Life, Knowlden and Rochelle Hudson did it once again in Les Misérables when they portrayed Cosette during different moments of the film.

Knowlden's face can be seen on most DVD covers of the film, which starred Fredric March and Charles Laughton.

[18] She was cast in Rainbow on the River (1936), starring Bobby Breen, one of the few children's films that Knowlden ever appeared in.

A reviewer from Detroit Free Press lauded her performance as Lucille Layton, calling her "a fine little actress".

She also played Laurie Martin, James Cagney's character's love interest, as a child in Angels with Dirty Faces the same year.

[7] She retired from acting in films in 1944 to concentrate on her educational studies, and developing her musical and dramatic skills.

She was attending the private California-based liberal arts and sciences college Mills College, majoring in music and drama, when she met and married the World War II Army Captain Richard Goates on July 30, 1946, at the All Saints Episcopal Church in San Leandro.

[23][24] Leaving the military and returning to the United States, Goates enrolled at Stanford University while Marilyn supported him through school.

[25][26] Throughout most of her post-Hollywood life, Knowlden was a musician, songwriter and playwright, authoring a number of songs and musical plays.

[28] In 2011, she published an autobiography she had written, Little Girl in Big Pictures, describing her experiences as a child actress during the golden years of Hollywood, and telling "the rest of the story" about her life.