Brooks, who had about 30 performances in films, was a B-movie leading lady during the 1930s and 1940s, with roles in such films as In Old Chicago (1937), Little Miss Broadway(1938) and the Shanghai Gesture (1941). In the late 1930s, she dated Cary Grant, who called her Brooksie, and rumors that the two would be married were circulated. Brooks, something of a socialite, also dated Howard Hughes.
She was reported (UK Sunday Telegraph December 1942) as being president of Parties Unlimited Inc. in an article about Hollywood at war.
Along with fellow actress Una Merkel, and accompanied by noted actor Gary Cooper, Phyllis was the first civilian woman to travel to the Pacific theater of war during World War II, on a USO tour. She was married to Torbert Macdonald, an 11-term Massachusetts Congressman who was John F. Kennedy's roommate at Harvard University, and who remained a close friend and confidant throughout his life. She moved to Cambridge, Massachusetts with her new husband in 1945 so he could complete his Harvard Law degree. Congressman Macdonald had been a Harvard football captain and a decorated PT boatcaptain in World War II. He died in office in 1976.
Brooks continued performing in summer stock theater after her marriage, and hosted the first television interview program in Boston in the early 1950s (on WBZ-TV). She retired from public performances after that, concentrating on raising her family. The couple had four children, the eldest of whom was President Kennedy's godson. She died on August 1, 1995 in Cape Neddick, Maine at the age of 80.