Kathryn Anderson was born in San Francisco in 1909. Her grandmother had emigrated to California from Norway in the late 19th century; both of Anderson's parents were native-born Americans.
Forbes was a radio scriptwriter and later began publishing short stories in national magazines. Her best known work was Mama's Bank Account, a fictionalized memoir, published in 1943, about a Norwegian family in 1920s San Francisco. Her heartwarming stories focused on their simple aspirations and their often difficult struggles, dreams and determinations, happiness, and sorrows.
In 1947, she published another novel, Transfer Point, regarding the daughter of divorced parents. Unlike Mama's Bank Account, for which she drew on the experiences of her Norwegian-born grandmother, the later novel was closer to Forbes' real-life childhood.