Disorders of sex development (DSDs) are congenital anomalies that affect sexual differentiation of genitourinary organs and secondary sex characters. A common cause of female genital virilization is congenital adrenal hyperplasia (CAH), in which excess androgen production during development of 46XX females can result in vaginal atresia, masculinization of the urethra, a single urogenital sinus, and clitoral hypertrophy or ambiguous external genitalia. Development of the vagina depends on sexual differentiation of the urogenital sinus ridge, an epithelial thickening that forms where the sex ducts attach to the anterior urethra. In females, the sinus ridge descends posteriorly to allow the vaginal opening to form in the vulva, whereas in males and in females with CAH, androgens inhibit descent of the sinus ridge. The mechanisms that regulate development of the female urethra and vagina are largely unknown. Here we show that the timing and duration of, and the cell population targeted by, androgen signaling determine the position of vaginal attachment to the urethra. Manipulations of androgen signaling in utero reveal a temporal window of development when sinus ridge fate is determined. Cell type-specific genetic deletions of androgen receptor (Ar) identify a subpopulation of mesenchymal cells that regulate sinus ridge morphogenesis. These results reveal a common mechanism that coordinates development of the vagina and feminization of the urethra, which may account for development of a single urogenital sinus in females exposed to excessive androgen during a critical period of prenatal development.