She wrote a number of screenplays for silent movies, but is best remembered for her groundbreaking exploits as "The World's Greatest Girl Reporter" during the 1920s and 1930s and her celebrity interviews for Photoplay magazine.

[3] She also wrote short stories for Cosmopolitan, The Saturday Evening Post, and other magazines and finished 9 of her 13 screenplays before returning to reporting for Hearst newspapers.

Writing in a distinctive, emotional style, St. Johns reported on, among other subjects, the controversial Jack Dempsey–Gene Tunney "long-count" fight in 1927, the treatment of the poor during the Great Depression, and the 1935 trial of Bruno Richard Hauptmann for kidnapping and murdering the son of Charles Lindbergh.

Her coverage of the assassination of Senator Huey Long in 1935, the abdication of King Edward VIII in 1936, the Democratic National Convention of 1940, and other major stories made her one of the best-known reporters of the day.

St. Johns again left newspaper work in 1948 to write books and to teach journalism at the University of California, Los Angeles.

During one Tonight Show visit, Paar noted that St. Johns had known many legends of Hollywood's Golden Age and was once rumored to have had Clark Gable's child.

"[7] In 1976, at the age of 82, she returned to reporting for the Examiner to cover the bank robbery and conspiracy trial of Patty Hearst, granddaughter of her former employer.