[6] Mathis was determined to become a screenwriter and, accompanied with her mother, she moved to New York City, where she studied writing and went to the movies in the evenings.

As one of the first screenwriters to include details such as stage directions and physical settings in her work, Mathis saw scenarios as a way to make movies into more of an art form.

[6] In 1921, Richard Rowland, the head of Metro, paid $20,000 and 10% of the gross earning for Vicente Blasco Ibáñez's novel The Four Horseman of the Apocalypse.

[9] The epic bestseller had been considered unadaptable by every major studio but Rowland handed the book to Mathis for adaptation and was so impressed with her screenplay that he asked her input on director and star.

The camera alights ever so briefly on what appears to be a pair of lesbians sitting together at the tango club, and features a scene with German officers coming down the stairs in drag.

Of the scene, Mathis later told the Los Angeles Times: "I had the German officers coming down the stairs with women's clothing on.

"[11] "She discovered me, anything I have accomplished I owe to her, to her judgment, to her advice and to her unfailing patience and confidence in me", said Valentino on Mathis in a 1923 interview with Louella Parsons.

When Valentino showed up on the set for The Conquering Power, another Mathis script with Rex Ingram at the helm, his new-found stardom went to his head, along with resentment at working for the same wage of $350 a week.

[13] The friction between him and Ingram, and his need for more money to support mounting debts, led Valentino to sign with Famous Players–Lasky (later known as Paramount Pictures) for $1,000 a week.

Mathis was also one of the people who helped bail Valentino out of jail when he was arrested for bigamy, having married Natacha Rambova without finalizing his divorce to Jean Acker.

When Mathis submitted a script for The Hooded Falcon, one of Valentino's pet projects, the couple deemed it unacceptable and asked to have it rewritten.

[7] Her strength lay in careful preparation of the shooting script along with the director, cutting out waste in production while at the same time sharpening narrative continuity.

When Erich von Stroheim presented Goldwyn Pictures with his masterpiece Greed (1924), following Frank Norris's novel McTeague very closely, it was 42 reels and 10 hours long.

Stroheim himself realized the original version was far too long, so he reduced it to 24 reels (6 hours), hoping the film could be screened with intermissions in two successive evenings.

MGM took Greed out of Stroheim's hands and gave it to Mathis, with orders to cut it even more, which she assigned to a routine cutter, Joseph W. Farnham.

Mathis remained at First National for two years, but left over limitations and signed with United Artists; with her husband she made one picture for them, The Masked Woman.

[6] A short woman with untamed brown hair and a love of Parisian fashion, she was also one of the first "writer-directors"[10] and laid the groundwork for the later development of screenwriters becoming producers.

[19] Mathis had been romantically linked to George Walsh and Rex Ingram; however, she returned from Italy engaged to an Italian cinematographer named Silvano Balboni.

[20] On July 26, 1927, during the third act of the Broadway show The Squall at the 48th Street Theatre while accompanied by her 81-year-old grandmother Emily Hawkes, Mathis suffered a fatal heart attack.