Born Corinne Anita Loos in Sisson, California (today Mount Shasta), where her father, R. Beers Loos, had opened a tabloid newspaper for which her mother, Minerva "Minnie" Smith did most of the work of a newspaper publisher. Loos had two siblings: Gladys, and Clifford (Harry Clifford), the latter being the eldest who would be a physician and co-founder of Ross-Loos Medical Group. The family moved to San Francisco in 1892, where Beers Loos bought still another newspaper, The Dramatic Event, a veiled version of a Police Gazette, with money Minerva borrowed from her father. On pronouncing her name, "The family has always used the correct French pronunciation which is lohse. However, I myself pronounce my name as if it were spelled luce, since most people pronounce it that way and it was too much trouble to correct them."

While living in San Francisco, Loos followed her dissolute alcoholic father as they explored San Francisco's underbelly; together they would sit on the pier, fishing and making friends with the natives, feeding into her lifelong fascination with lowlifes and loose women. In 1897, at their father's urging, she and her sister performed in the San Francisco stock company production of Quo Vadis. Gladys died while their father was on one of his drinking and philandering "fishing trips". Anita continued appearing on stage, sometimes being the family's sole breadwinner. Eventually Beers Loos' spendthrift ways caught up with them, and in 1903, Beers Loos took an offer to manage a theater company in San Diego. There, Anita performed simultaneously in her father's stock company, and under another name with the more legitimate stock company in town. It was around this time that she started shaving years off her true age.

Loos had known she wanted to be a writer since she was six, and she also wanted to free herself of the shackles of stock performance. After graduating from high school, Loos devised a method of cobbling together published reports of Manhattan social life, mailing them to a friend in New York who would submit them under their own name for publication in San Diego. Her father had turned out some one-act plays for the stock company, and encouraged Anita to work in the field herself. She wrote The Ink Well, a successful piece for which she would receive periodic royalties.

In 1911, the theater was running one-reel films after each night's performances; Anita would take a perfunctory bow and run to the back of the theater to watch them. She sent her first attempt at a one-reel screenplay, The New York Hat, to the Biograph Company, for which she received $25. The New York Hat, starring Mary Pickford and Lionel Barrymore and directed by D. W. Griffith, was her third screenplay and the first to be produced. Loos dredged real life and real situations for her scenarios, she dished up her father's cronies, her brothers friends, the rich vacationers from the San Diego resorts, eventually every experience became grist for her script mill. By 1912, Loos had sold scripts to both the Biograph and Lubin Companies. Between 1912 and 1915 she turned out 105 scripts, only four of which went unproduced, and she would write 200 scenarios before she ever saw the inside of a studio.

Her mother had objected to Loos' working in Hollywood. In 1915, trying to escape her influence, Loos married her first husband Frank Pallma, Jr., the son of the band conductor. But Frank proved to be penniless and dull – after six months, Anita sent him out for hair pins – while he was gone she packed her bags and went home to her mother. After that Minnie rethought her position on a Hollywood career. Afterwards, accompanied by her mother, Anita joined the film colony in Hollywood where Griffith put Loos on the payroll for Triangle Film Corporation at $75 a week with a bonus for every produced script, perhaps making her the first "staff writer". Many of the scripts she turned out for Griffith went unproduced, some he considered unfilmable because the "laughs were all in the lines, there was no way to get them onto the screen", but he encouraged her to continue, because he liked reading them for amusement. Her first screen credit was for an adaptation of Macbeth in which her billing came right after Shakespeare's. When Griffith asked her to write the subtitling for his epic Intolerance (1916), she traveled to New York City for the first time to attend its premiere. Instead of returning to Hollywood, Loos spent the fall of 1916 in New York and met with Frank Crowninshield of The New Yorker. They had an instant rapport and Loos would remain a New Yorker contributor for several decades.

Loos returned to California just as Griffith who wanted to make longer films, was leaving Triangle, and she joined director and future husband John Emerson for a string of successful Douglas Fairbanks films. Loos and company realized that Douglas Fairbanks' acrobatics were an extension of his effervescent personality and parlayed his natural athletic ability into swashbuckling adventure roles. His Picture in the Papers (1916) was noted for its wry style of discursive and witty subtitles: "My most popular subtitle introduced the name of a new character. The name was something like this: 'Count Xxerkzsxxv.' Then there was a note, 'To those of you who read titles aloud, you can't pronounce the Count's name. You can only think it.' " The five films Loos' wrote for Fairbanks made him a star. When Fairbanks was offered a sweetheart deal with Famous Players-Lasky, he took the team of Emerson-Loos with him at the high income of $500 a week. During this time Loos, Fairbanks and Emerson collaborated well together and Loos was getting as much publicity as either Lillian Gish or Pickford. Photoplay magazine labeled her "The Soubrette of Satire." In 1918, Famous Players-Lasky offered the couple a four-picture deal in New York for more money than they had been making with the Fairbanks unit.

Loos, Emerson and fellow writer Frances Marion, migrated to New York as a group, Loos and Emerson sharing a leased mansion in Great Neck, Long Island. Loos desperately wanted Marion as chaperone, as she found herself attracted to Emerson. He would readily admit that he "had never been, nor could be, faithful to any one female." Loos, convinced herself that he would see that she was different than all his other girls, and that behind the outwardly dull exterior was a great mind. She would be wrong on both counts. She would later write: "I had set my sights on a man of brains, to whom I could look up", she lamented, "but what a terrible let down it would be to find out that I was smarter than he was."

The pictures for Famous Players-Lasky were not as successful as their previous films, partly because they starred Broadway headliners not adept at screen acting. In addition to their film "collaborations" the couple wrote two books: Breaking Into the Movies, published in 1919 followed by How to Write Photoplays in 1921. Though the scripts carried both names, they were mostly products of Loos alone. Later Loos would claim that Emerson took all of the money and most of the credit for projects, even though his contribution usually consisted of observing from bed as Loos worked. Much to the chagrin of her friends, her adoration of Emerson had manifested as subservience. When their contract was not renewed he blamed her scripts though he had claimed credit for them. When William Randolph Hearst offered Loos a contract to write a picture for his mistress Marion Davies, Loos included Emerson in the deal, though his presence was unnecessary. Hearst liked the picture and Getting Mary Married (1919) was one of the few Marion Davies pictures that didn't lose money.

Loos and Emerson turned down another picture with Davies, preferring to write for their old friend Constance Talmadge, whose brother-in-law Joseph Schenck (husband of Norma Talmadge)was an independent producer. Both A Temperamental Wife (1919) and A Virtuous Vamp (1919) were great hits for Talmadge. The Schenck studios filmed in a New York warehouse and Loos and Emerson occupied suites at the Algonquin. Individually Anita liked many members of the Algonquin Round Table, but as a group she found them overwhelming. In the spring of 1919, the couple joined the Talmadges and the Schencks at the Ambassador Hotel on Park Avenue, with Constance, filling the void left by the loss of her sister many years before. When Anita and Constance weren't working, they went shopping.[citation needed] The Talmadge-Schencks convinced Anita to summer with them in Paris without Emerson. Much of this adventure would end up as fodder for Loos's book Gentlemen Prefer Blondes.

When they returned they produced five more films in sixteen months. Emerson still received his full salary though reputedly made few appearances on set and the script credit continued to name both of them. Emerson's assistant, who had taken up the workload on set, objected to the lack of credit and unfair reimbursement and was subsequently replaced. The new assistant director had eyes for Loos, who had filed for divorce from her estranged first husband. Emerson proposed marriage. They were married at the Schenck estate on June 15, 1919. Loos was among the first to join Ruth Hale's Lucy Stone League, an organization that fought for women to preserve their maiden names after marriage. Hale, wife of playwright Heywood Broun had struggled to get a U.S. passport issued in her birth name.