Mildred J. Hill was an American composer and pianist, best known for co-writing the popular children's song "Happy Birthday to You" with her sister, Patty Hill. Born on June 27, 1859, in Louisville, Kentucky, she was a prominent figure in early childhood education and music. In addition to her work on "Happy Birthday," Mildred J. Hill composed various songs and contributed to educational music. She was also involved in the development of the Patty Hill School, which focused on progressive education methods. Her contributions to music and education have left a lasting impact.