Before going into radio, Gregg played the double bass with the Pasadena Symphony and Pops. She was a member of the Singing Strings group heard initially on KHJ in Los Angeles in 1937 and later on CBS and Mutual.

Gregg married Producer Jaime del Valle in 1948, (Another source says October 15, 1947, in Las Vegas, Nevada.) and they had three children, Gregg, Jaime, and Ricardo. They were divorced December 22, 1959.

Gregg supplied the voice of "Mrs. Bates" in Psycho as did Jeanette Nolan and Paul Jasmin, all uncredited. Only Gregg did the voice in the sequels Psycho II and Psycho III. She voiced "Tarra" on the 1967 animated TV series The Herculoids. She reprised that role when the series was revived in 1981 as part of the Space Stars animated series.

Gregg once said of her work as a character Actress on television: "I work steadily, but I have no identity." She added, "When casting people have a call for a woman who looks like the wrath of God, I'm notified." On television, Gregg appeared in nearly every narrative television series in the late '50s through the early 1970s, including Bourbon Street Beat, Hawaiian Eye, 77 Sunset Strip, The Rockford Files, Maverick, The Virginian, Alfred Hitchcock Presents, Wagon Train, Trackdown, Make Room for Daddy, Philip Marlowe, My Favorite Martian, Twilight Zone, Hazel, Kung Fu, Gunsmoke and Bewitched,Bonanza. She portrayed Jessie Copperton, a domineering cattle ranch matriarch, on Season 14; Episode 15 of Gunsmoke, entitled "The Twisted Heritage", in which her character has designs on welcoming Miss Kitty (Amanda Blake) into the family as a replacement for her former daughter-in-law. Gregg may be best remembered for her many appearances in Dragnet. Jack Webb used her in dozens of roles on both the radio and TV versions of the show, as well as in the 1954 film version of Dragnet. In later years, she appeared on other shows produced by Webb's production company, Mark VII Limited (e.g., Adam-12, Emergency!). Gregg also played non-recurring character roles in four episodes of the long-running CBS series Perry Mason, including the 1958 episode, "The Case of the Cautious Coquette," and "The Case of the Pathetic Patient" from 1961.

Gregg died from lung cancer in Encino, California, September 15, 1986, aged 70. She was survived by her three sons: Gregg, Jaime and Ricardo del Valle.

On the radio series Have Gun–Will Travel (starring John Dehner as Paladin), Gregg portrayed Miss Wong (the girlfriend of Hey Boy), and also appeared in very different roles in the concurrent television series with Richard Boone. She had the role of Betty Barbour on One Man's Family and played Richard Diamond's girlfriend, the wealthy Helen Asher, on the radio series Richard Diamond, Private Detective (starring Dick Powell as Diamond). She later guest-starred in an episode of the television version of Richard Diamond, starring David Janssen.