Girl before a Mirror shows Picasso's young mistress Marie-Thérèse Walter, one of his favorite subjects in the early 1930s. Her white-haloed profile, rendered in a smooth lavender pink, appears serene. But it merges with a more roughly painted, frontal view of her face—a crescent, like the moon, yet intensely yellow, like the sun, and "made up" with a gilding of rouge, lipstick, and green eye-shadow. Perhaps the painting suggests both Walter's day-self and her night-self, both her tranquillity and her vitality, but also the transition from an innocent girl to a worldly woman aware of her own sexuality.
It is also a complex variant on the traditional Vanity—the image of a woman confronting her mortality in a mirror, which reflects her as a death's head. On the right, the mirror reflection suggests a supernatural x-ray of the girl's soul, her future, her fate. Her face is darkened, her eyes are round and hollow, and her intensely feminine body is twisted and contorted. She seems older and more anxious. The girl reaches out to the reflection, as if trying to unite her different "selves." The diamond-patterned wallpaper recalls the costume of the Harlequin, the comic character from the commedia dell'arte with whom Picasso often identified himself—here a silent witness to the girl's psychic and physical transformations.
from The Museum of Modern Art, MoMA Highlights, New York: The Museum of Modern Art, revised 2004, originally published 1999, p. 161