Agnes Christine Johnston (January 11, 1896 – July 19, 1978) was an American screenwriter.
Johnston was born in Swissvale, Pennsylvania. She wrote for 84 films between 1915 and 1948. She had intended to go to college, but was sufficiently proficient in writing scenarios for films following graduating from high school that she ended up taking just a single course in writing plays at Harvard in 1918. When Vitagraph Studios gave her an assignment to write a scenario for a James Oliver Curwood novel God's Country and the Woman, she completed the scenario for the William Duncan and Nell Shipman 1916 film within 24 hours.
A number of Andy Hardy films starring Mickey Rooney were written by Johnston, among them The Hardys Ride High, Life Begins for Andy Hardy, The Courtship of Andy Hardy, Andy Hardy's Double Life and Andy Hardy's Blonde Trouble. Her other films included the coming-of-age comedy Seventeen, the romantic comedy Janie and its sequel, Janie Gets Married, plus the 1946 adaptation of the classic Black Beauty.