She became the first female screenwriter to be nominated for the Best Original Screenplay Oscar when the category was introduced in 1940, and was the first screenwriter to receive two Academy Award nominations in the same year in separate categories, for co-writing the screenplay for the films Foreign Correspondent (1940) (original) and Rebecca (1940) (adapted), both directed by Alfred Hitchcock, with whom she had a long professional relationship.

Harrison appears in a scene in Hitchcock's original version of The Man Who Knew Too Much (1934), eating dinner with Peter Lorre's character.

[3] When Hitchcock moved to Hollywood in March 1939 to begin his contract with David O. Selznick to direct films, Harrison emigrated with him as an assistant and writer.

[1] She continued contributing to the screenplays for Hitchcock's films Rebecca (1940), another du Maurier adaptation, Foreign Correspondent (1940), Suspicion (1941), and Saboteur (1942).

She was also credited as one of the screenwriters for Dark Waters (1944) after Phantom Lady star Franchot Tone persuaded her to work on the script as the writer of the original story, Marian Cockrell, was having difficulties with the adaptation.