In 1937, Day debuted onscreen in a bit part in Stella Dallas. Shortly afterwards, she won lead roles in several George O'Brien Westerns at RKO Pictures, in which she was billed as Laraine Hays and then Laraine Johnson.

Day was born in Roosevelt, Utah, one of eight children in an affluent Mormon family. She had a twin brother, Lamar. The family later moved to California, where she began her acting career with the Long Beach Players. She was a 1938 graduate of Polytechnic High School in Long Beach, California.

In 1939, she signed with Metro-Goldwyn-Mayer, and became popular and well-known (billed as Laraine Day) as Nurse Mary Lamont, the title character's fiancee in a string of seven Dr. Kildare movies beginning with Calling Dr. Kildare (1939), with Lew Ayres in the title role.

Her roles for other studios were often far more stimulating than those MGM gave her, including a prominent supporting part in the Irish melodrama My Son, My Son! (1940). She also starred in the Alfred Hitchcock thriller Foreign Correspondent (1940) with Joel McCrea, and the psychological mystery The Locket (1946) with Robert Mitchum, Brian Aherne, and Gene Raymond. In 1941, she was voted the number one "star of tomorrow" in Hollywood.

On January 21, 1947, Day traveled to Juarez, Mexico, where she received a second divorce decree. Later that day, she traveled to El Paso, Texas, where she married baseball manager Leo Durocher. Upon returning to California, the judge who granted Day's interlocutory divorce from Hendricks stated that the Mexican divorce she received was not legal, and since she failed to wait the one-year period for her divorce to become final, deemed her Texas marriage illegal, as well. After waiting about a year, Day and Durocher remarried on February 16, 1948, in Santa Monica, California. During her marriage to Durocher, Day was often referred to as "The First Lady of Baseball". While Durocher was managing the New York Giants, she wrote the book Day With the Giants (1952). In 1952, she wrote and published another book entitled The America We Love. She was also the host of Day With the Giants, a 15-minute television interview program broadcast before New York Giants home games. Day and Durocher divorced in June 1960.

She was paired opposite major film stars, including Lana Turner, Cary Grant, and John Wayne, and hosted a television show alternately called Daydreaming with Laraine or The Laraine Day Show (1951). During a time when she had a break in her film career, she made her stage debut opposite Gregory Peck in the national theatre tour of Angel Street. She also made other stage appearances in Lost Horizon, the 1973 revival of The Women, and a revival of The Time of the Cuckoo. In the 1940s, she made guest appearances on radio in both Lux Radio Theatre and The Screen Guild Theater.

In October 1960, Day appeared in the Nixon-Lodge Bumper Sticker Motorcade Campaign in Los Angeles along with Ginger Rogers, Cesar Romero, Irene Dunne, Dick Powell, Mary Pickford, and John Payne.

In 1961, she appeared with the Mormon Tabernacle Choir in their production of Let Freedom Ring, which was an inspirational one-hour program dedicated to the spirit of American Freedom.

Day was, as she described herself, "very much a Republican". She was a vocal supporter of Richard Nixon, whom she later met at the 1968 Republican National Convention, citing him to be the type who would "go out of his way to help the American people". She also supported Dwight D. Eisenhower in 1952 and longtime Hollywood friend, and former co-star, Ronald Reagan in the 1980 and 1984 presidential elections, saying of both Nancy Reagan and him adoringly, "Ronald Reagan makes me proud to be an American. His intelligence, capability, and Christian brotherhood are so inspiring and his way of leadership is just superb. I consider myself lucky to have been his leading lady in The Bad Man and a short-subject reel and as a nation all together we are beyond fortunate to have the leadership of such fine people as the Reagan's."

In the 1970s, she was the spokesperson for the Make America Better campaign and traveled across the country sharing her views on environmental issues. In that same time, she was active in establishing a playhouse in Los Angeles for Mormon actors and she helped bring Ray Bradbury's work to the attention of the public.

Her body was taken back to California, and on November 15, 2007, a memorial Service was held at Forest Lawn Memorial Park in the Hollywood Hills.