Blyth was born to parents who divorced shortly after her birth. She has been a devout Catholic her whole life. Blyth began her acting career initially as "Anne Blyth," but changed the spelling of her first name back to "Ann" at the beginning of her film career. Her first acting role was on Broadway inWatch on the Rhine (from 1941 until 1942). She was signed to a contract with Universal Studios, and made her film debut in Chip Off the Old Block in 1944. In musical films such as Babes on Swing Street, and Bowery to Broadway (both 1944), she played the part of the sweet and demure teenager.
On loan to Warner Brothers Blyth was cast "against type" as Veda Pierce, the scheming, ungrateful daughter of Joan Crawford in the 1945 filmMildred Pierce. Her dramatic portrayal won her outstanding reviews and she received a nomination for an Academy Award for Best Supporting Actress. (Crawford won the Best Actress award for that film.)
Having injured her back after Mildred Pierce, Blyth was not able to capitalize on its success completely, although she was still able to make a few films. She played the part of Regina Hubbard in Another Part of the Forest (a 1948 prequel to The Little Foxes), and achieved success playing amermaid in Mr. Peabody and the Mermaid. Her other films include : Our Very Own (with Farley Granger), The Great Caruso (with Mario Lanza), One Minute to Zero (with Robert Mitchum), The World in His Arms (with Gregory Peck), Rose Marie, The Student Prince, Kismet, The Buster Keaton Story, and The Helen Morgan Story (with Paul Newman). Even though her voice was more like the original Helen Morgan, her vocals were dubbed by Gogi Grant, a popular singer at the time. That soundtrack was much more successful than the film itself. It also turned out to be Blyth's final film role.
During the late 1950s and 1960s Blyth worked in musical theater, summer stock, and television, including a starring role in a 1960 adaptation of A. J. Cronin's The Citadel. She guest-starred on October 8, 1958, on NBC The Ford Show, Starring Tennessee Ernie Ford, the episode in which the 1959 Ford vehicles were introduced to the public.She appeared as Martha in Suspected in December 1959 in the CBS anthology series, The DuPont Show with June Allyson. Blyth also became the spokesperson for Hostess Cupcakes. Her last television appearances were in episodes of Quincy, M.E. in 1983 and Murder, She Wrote in 1985.
Blyth has a star on the Hollywood Walk of Fame at 6733 Hollywood Boulevard for her contribution to motion pictures.
Blyth married James McNulty, brother of singer Dennis Day, in 1953; the couple had five children. She is known as one of the more famous residents of the celebrity enclave of Toluca Lake, California. In 1973, she and McNulty, both of whom were Catholic, received the rank of Lady and Knight of the Holy Sepulchre by Cardinal Cook. McNulty died May 13, 2007 in La Jolla.
In the December 1952 edition of Motion Picture and Television Magazine Ann Blythe stated in an interview that she endorsed Dwight D. Eisenhower for president the month before in the 1952 presidential election.