We conducted a systematic review to generate a comprehensive understanding of cisgendered girls' mathematical identity within the United States to better understand: (a) how mathematical identity is associated with girls' participation, engagement, and achievement in mathematics; (b) the factors related to girls' mathematical identity; and (c) strategies for developing strong mathematical identities within girls. We identified 76 articles that examined mathematical identity in young girls and women. Our review suggests that positive girls' mathematical identity was consistently linked with increased performance, participation and persistence in mathematics. Factors and strategies affecting girls' mathematical identity included the role of parental attitudes and beliefs; societal and cultural influences; and the intersections of sex and race. Practical implications for researchers and practitioners are also discussed.