[1][2] She attended Immaculate Heart Convent School in Los Angeles,[3] studying first with Daniel Frohman[4] and then with Richard Boleslawski at the American Laboratory Theatre in New York City during the early 1920s.

[3] Nelson made her stage debut in New York on April 4, 1928, at the Laboratory Theatre under Boleslawski's direction, portraying the title character in Jean-Jacques Bernard's Martine.

Over the next two seasons, Nelson made two more appearances—in Checkhov's The Seagull and Vladimir Kirshon's Red Rust[5]—prior to becoming, in 1931, a charter member of the newly formed theatre collective, The Group Theatre, with whom she remained throughout its run from 1931 to 1941, receiving particular praise for her performance as the chief striker's wife in Clifford Odets' play, Waiting for Lefty.

As her career began to take off, she was compelled to put things on hold when her husband, the director John Cromwell, a leading Roosevelt Democrat in the film industry, was falsely accused of Communism by actor Adolphe Menjou in front of the House Un-American Activities Committee hearings on Hollywood in 1951[7] and his career went on to be blacklisted.

The following year, she played Aunt Beatrice Sloan Cory and Cromwell portrayed the befuddled Bishop Martin in A Wedding, a comedy directed by Altman.

[9] Moreover, as early as 1968, Nelson had performed onstage under her stepson's direction, giving a well-received performance as Mary Tyrone in a regional production of O'Neill's Long Day's Journey Into Night;[10][11] reprising the role she'd first played on Broadway in 1957, initially as Florence Eldridge's understudy, and then as the permanent replacement for an ailing Fay Bainter during the show's national tour.

[18]Nelson's final feature film appearance was in 1990's Awakenings; her performance—as the mother of a hospital patient played by Robert De Niro[19][20] (a role which—in a widely disseminated contemporaneous story published by Premiere Magazine—was erroneously reported as having gone to an Oscar-flaunting Shelley Winters)[21]—was singled out for praise by several critics,[22] including the Wall Street Journal's Julie Salamon: "Nelson achieves a wrenching beauty that stands out even among these exceptional actors doing exceptional things.

She was a New York stage actress in the 1930s who transitioned to movies but was blacklisted in the 1950s when her second husband was among those Senator Joseph McCarthy labeled a Communist.