She primarily worked in television, guest starring in nearly 100 prime time shows, and played continuing roles on two daytime soap operas, Dark Shadows and The Secret Storm.

Every year during high school summer vacation, she appeared in summer stock productions, playing leading or featured roles in classic stage plays such as Our Hearts Were Young and Gay; The Girl on the Via Flaminia; Come Back, Little Sheba; Time of the Cuckoo; The Seven Year Itch;[3] Ladies in Retirement; Bell, Book and Candle; Time Out for Ginger; Picnic; The Little Foxes; Tobacco Road; and Life With Father.

Her filmed television credits include guest star roles on most of the major shows that were running during the late 1950s and throughout the 1960s, including Stagecoach West, Father Knows Best, The Tab Hunter Show, My Three Sons, The Americans, Gunsmoke (as title character in the episode "Melinda Miles" - 1957 S6E37), Bonanza, The Virginian, Arrest and Trial, 77 Sunset Strip, Rawhide, Tales of Wells Fargo, Wagon Train, Laramie, Route 66, Hawaiian Eye, The Rifleman, Thriller, Maverick (in the episode "Dodge City or Bust" with Jack Kelly and a brief appearance by Roger Moore), The Life and Legend of Wyatt Earp, Dobie Gillis, Sam Peckinpah's The Westerner, and The Man from U.N.C.L.E.. Millay made three television pilots for prospective new television series, Slezak and Son, Boston Terrier, and Las Vegas Beat.

[8] After completing Paramount's Tarzan and the Great River opposite Mike Henry and Jan Murray that was shot in Brazil,[9][10] executive producer Dan Curtis offered her the contract role of "Laura Collins" on his ABC-TV daytime series, the cult classic Dark Shadows in November 1966.

[11] She went on to appear in sixty-two episodes,[12] and became the show's first supernatural character, playing an immortal phoenix-woman who is burned in a fire and reborn to spend another century on Earth.