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 died in San Francisco in 1966. She is interred at Holy Cross Catholic Cemetery in Colma. (Section 5, Row 8, Grave 35), which is just south of the city where she was born and raised.

Forbes was a radio scriptwriter and later began publishing short stories. Forbes published the first two chapters of her novel, Mama's Bank Account and Mama's Roomer in 1942. Mama's Bank Account, her best known work was published in 1943 about a Norwegian family living in 1920s San Francisco. Her heartwarming stories focused on their simple aspirations and their often difficult struggles, dreams and determinations, happiness, and sorrows.

Forbes' book served as the inspiration for John Van Druten's 1944 play I Remember Mama. RKO Pictures bought the rights to the novel and later bought fifty percent of John van Druten's Broadway adaptation and made a deal with the play's producers, Richard Rodgers and Oscar Hammerstein II. RKO Pictures distributed the 1948 film, which was directed by George Stevens and starred Oscar Homolka, Edgar Bergen, Irene Dunne, Barbara Bel Geddes and Ellen Corby.

A popular long-running television series was broadcast on the CBS network from July 1, 1949 to March 17, 1957. In addition to the John van Druten adaptation, McLean's novel was twice turned into a stage musical. The first was adapted by Neal Du Brock and John Clifton and opened in Buffalo, New York in 1972, with Celeste Holm in the role of Mama. In 1979, Richard Rodgers wrote the music for the second musical version for which Norwegian actress Liv Ullmann played the role of Mama.

In 1947, Forbes 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.