Glynis Johns, an actress who became a British film star in the late 1940s playing a flirty mermaid named Miranda, portrayed a singing suffragist in the Disney musical “Mary Poppins” and won a Tony Award in the musical “A Little Night Music,” where she introduced Stephen Sondheim’s standard “Send in the Clowns,” died Jan. 4 at an assisted-living home in Los Angeles. She was 100.