After marrying playwright Benn Wolf Levy in 1933, Cummings more or less abandoned her Hollywood career for the stage and England. Her film appearance became infrequent, although she shone as Robert Montgomery's mystery writer spouse in "Busman's Honeymoon/Haunted Honeymoon" (1940) and had perhaps her best screen role as Rex Harrison's second wife Ruth in the delightful screen version of Noel Coward's frothy "Blithe Spirit" (1945). Her later screen roles found her cast as a demanding prima donna in "The Intimate Stranger/Finger of Guilt" (1956), the aunt of a boy trekking through Africa in "Sammy Going South/The Boy Ten Feet Tall" (1963) and second lead to Angela Lansbury in "In the Cool of the Day" (1963), her last films made directly for the big screen.

Cummings, instead, concentrated on her family and a celebrated stage career that saw her in London and in NYC in such memorable roles as Emma Bovary in "Madame Bovary" (1937), the nagging wife in "The Shrike" (1953), Martha in "Who's Afraid of Virginia Woolf?" (1964) and Gertrude to Nicol Williamson's "Hamlet" (1969, although Judy Parfitt replaced her in the film). Joining Laurence Olivier's National Theatre in the 70s, Cummings triumphed as Mary Tyrone in "Long Day's Journey Into Night" (1971, filmed for TV by ABC). But the crowning achievement in her long career came with her effective and moving portrayal of a former daredevil aviatrix who suffers a stroke in Arthur Kopit's "Wings" (1979), for which she received a Best Actress Tony Award (in a tie with Carole Shelley). After repeating that role for a 1983 PBS version, she starred in the Off-Broadway revival of "The Chalk Garden" (1982), starred as Amanda Wingfield in a Florida production of "The Glass Menagerie" (1984), toured the UK in a one-woman show about actress "Fanny Kemble" and made her last TV appearance in the elegiac British drama "Love Song" (aired in the USA on PBS in 1987).