[2] With her mother's support, but her father's disapproval, she trained with the Lyric Stage Academy in London, before making her professional debut in Johannesburg at the age of 20.

Throughout her career, her first love was the theater; and, as the years passed, she appeared less frequently in London and became a frequent performer on Broadway, appearing in such plays as A Successful Calamity (1917), A Little Journey (1918), Spring Cleaning (1923), The Distaff Side (1934), The Importance of Being Earnest (which she also directed, 1939), When We Are Married (1939), Ladies in Retirement (1940), The Pirate (1942), Ten Little Indians (1944), Lady Windermere's Fan (1947), and The Madwoman of Chaillot (1948).

Her other film credits include Darby O'Gill and the Little People (1959), The Misfits (1961), The Magic Sword (1962), The Notorious Landlady (1962), Dead Ringer (1964), Camelot (1967) and The Producers (1967).

Her other work for television included guest roles in Dennis the Menace, The Twilight Zone as the elderly wife of the ageless title character in "Long Live Walter Jameson", Thriller, Dr. Kildare, The Man from U.N.C.L.E., The Name of the Game, Bewitched, Batman, Love, American Style, Cannon, Police Story, The F.B.I., and the last episode of Perry Mason, titled "The Case of the Final Fade-Out", in which she played an aging actress who ended up as a second defendant.

Winwood's final film appearance, at age 92[4] in Murder by Death (1976), was as an ancient nursemaid to Jessica Marbles (a spoof of Miss Marple, played by Elsa Lanchester).

When she took on her final major television role in a 1979 episode of Quincy, she officially became, at age 96, the oldest actor working in the U.S., narrowly beating fellow British actress Ethel Griffies.

Winwood appeared as a character in Answered Prayers, Truman Capote's final, unfinished, thinly veiled roman à clef.