Johns' spirited personality became her signature, enlivening whatever material she chose. A vivacious delight in her Oscar-nominated supporting turn as a hotelkeeper who sets her sights on a matrimonially-evasive Peter Ustinov in the Australian-set "The Sundowners" (1960), she may be best remembered as the nutty suffragette wife of David Tomlinson and mother of Julie Andrews' charges in the Disney tour de force "Mary Poppins" (1964). A standout as James Stewart's practical wife in "Dear Brigette" (1965), Johns opted for tongue-in-cheek villainy as Lady Penelope Peasoup in several 1967 episodes of the popular ABC series "Batman". Always proud of her Welsh roots, she particularly enjoyed playing opposite Richard Burton in the film version of Dylan Thomas' "Under Milkwood" (1971), and as she matured, her spunky persona lent itself readily to eccentric grandmothers in "The Ref" (1994), "While You Were Sleeping" (1995) and "Superstar" (1999). She also starred in her own short-lived sitcom, "Glynis" (CBS, 1963), and returned as a series regular in "Coming of Age" (CBS, 1989-90), playing the perpetually perky Trudie Pepper.
Never one to stay away from the theater for long, Johns starred on the London stage at the age of 19 in the title role of "Peter Pan" (1943), made her Broadway debut as "Gertie" in 1952 and was back on the Great White Way as George Bernard Shaw's "Major Barbara" (1956) in a production directed by Charles Laughton. Her most memorable Broadway turn came as Desiree Armfeldt, the leading lady of the Hugh Wheeler-Stephen Sondheim hit musical "A Little Night Music" (1973). After a particular scene was staged by Harold Prince, Sondheim, realizing a song was required for her character, tossed off the contemporary standard "Send in the Clowns", which the composer tailored for Johns' particular vocal qualities (what Sondheim called "a rumpled bed"). During the Boston run of the show prior to its New York opening, Johns fell ill and there was speculation that another actress would assume the role. Fiercely protective of the part, Johns declaimed that no one else would sing her songs and recovered sufficiently to score a personal triumph and a Tony Award as Lead Actress in a Musical. The actress returned to Broadway in a 1989 revival of the Somerset Maugham play "The Circle", starring opposite Rex Harrison and Stewart Granger, and later acted in a 1991 Los Angeles revival of "A Little Night Music", this time taking the part of Madame Armfeldt, the mother of the character she had originated. (Desiree was played by Lois Nettleton.) Johns later starred in a 1998 Long Island staging of Horton Foote's play "A Coffin in Egypt", playing a 90-year-old grand dame reminiscing about her life on and off a Texas ranch.