Union Square began as part of Willowbrook, the John Donnell Federal-period estate, which he purchased in 1802 from Baltimore merchant and later Mayor Thorowgood Smith. In 1847, the Donnell family heirs donated the two-and-a-half-acre lot in front of the manor house to the City of Baltimore to be designated as a public park. Beginning in the 1850s, the Donnell family started to work with a number of speculative builders to develop the neighborhood.
In 1867, the Donnells left Willowbrook (now the site of Steuart Hill Academic Academy), and the house was given to the Sisters of the Good Shepherd. The building served as a convent and home for wayward girls until its demolition in the mid-1960s. The oval dining room was removed from the mansion and recreated in the Baltimore Museum of Art where it remains a part of the American Decorative Arts wing.
This demolition sparked a renewed awareness of historic places and their importance to the community, as residents organized to form the Union Square Association and received historic district designation for the area in 1970.