D. W. Griffith directed the resulting film, The New York Hat (1912), and she worked regularly for Biograph until 1916, during which time she wrote over 100 scenarios and the accompanying title cards. Known for her wit, in 1916 when she wrote the film adaptation of MacBeth, the credits read “by William Shakespeare and Anita Loos". She is credited with being the first person to use verbal humor on title cards.

Loos is perhaps best remembered as the author of Gentlemen Prefer Blondes (1928), which was made into two films and spawned a se-quel-of-sorts in Gentlemen Marry Brunettes (1955). In 1926 she also wrote the play The Whole Town's Talking, which she later adapted as a film for the screen, titled The Ex-Bad Boy (1931).

In addition to her work writing plays and scripts, Loos wrote several books on her life and career, including A Girl Like7(1966), Kiss Hollywood Good-Bye (1974), and Cast of Thousands (1977). She also wrote two books about working in Hollywood, titled How to Write Photoplays (1920) and Breaking Into the Movies (1921). Although she supposedly wrote the books in collaboration with her (then) husband John Emerson, later it was discovered that he often made her put his name to works that he did no actual work on.

Although she was an independent woman earning her own way in a “man's world,” Loos never aligned with the feminist movement. Her writing, while amusing and satirical, never placed women in roles that were not socially acceptable. Whether she actually believed that women should be subservient to men is not clear, but controlling men populated her personal life.

Less remembered is the fact that Loos produced several films in the last years of the 1910s. Her first production effort was Come On In (1918), a film that she also wrote. In the next two years she produced five other films and then, inexplicably, returned to screen-writing.