Until her mid-twenties, Helen's life was dictated by George Tyler, who had become her producer. She was not allowed to socialize and attend parties with the other actors and actresses, which severely hampered her ability to meet new friends and date men. But in 1924, Helen was forced to choose between Tyler's self-centered demands and her own values. He had pushed her to join the Fidelity League, a union that existed mainly to proclaim loyalty to producers. However, Helen chose the Actors' Equity union, a rival union that that had been formed to try and raise the meager salaries of stage actors. This broke the ultimatum that Tyler had given her, and freed Helen from his tight hold on her life.
This led to the introduction of her to Charlie MacArthur, a Chicago journalist and playwright. Helen was invited by an actor friend to a high-society gathering where her demure disposition did not fit in well with the boisterous crowd. While she sat in the corner thinking of a good excuse for leaving, a handsome, green-eyed gentleman came over and offered her some peanuts. As he put them in her hand, he said, "I wish they were emeralds." Helen was in love. Many people told her their relationship wouldn't work, saying he was too much of a prankster and cynic for her innocent personality. But they both possessed a childlike charm, and were happily married until Charlie's death in 1956. The book Front Page Marriage discusses their lives together in detail.
Helen's first child was born amidst a somewhat humorous controversy. While she was pregnant, Helen was the main character in the play Coquette. She became very sick and was forced to quit the production, leaving the director to close the play altogether. The other actors were owed severance pay, but producer Jed Harris attempted to get out of it by saying Helen's reason for leaving was an "Act of God" that he couldn't control. Eventually he had to pay the actors, but Mary MacArthur would always be remembered as the "Act of God Baby." Helen and Charlie adopted a son, James, in 1939. Jamie, as Helen sometimes called him, went on to star in the long-running television series Hawaii Five-O