In 1949, the Pittsburgh Regional Planning Association decided to build an underground parking garage with a rooftop park. This idea was inspired by R.K. Mellon, who had seen a similar development in San Francisco's Union Square. Mellon family foundations offered four million dollars for the project, just over half of the project costs. The park is a memorial to two Mellon brothers, Andrew W. Mellon and Richard B. Mellon.
The garage was designed by Mitchell & Ritchey. The 1.37-acre park is by Simonds & Simonds. Main features include 25,000 plants, trees, and shrubs planted in containers throughout the park, terrazzo paving that references Pittsburgh's Golden Triangle, and several fountains, including a cast bronze fountain. The park was renovated in 2013 by Pittsburgh Parks Conservancy.
Simonds & Simonds was the firm established by renown modernist landscape architect, John Ormsbee Simonds (1913-2005) and his brother, Philip. Simonds' firm, which would later be known as the Environmental Planning and Development Partnership was responsible for the design of several notable projects. Additional landscapes designed by the firm can be seen on the North Side at Lake Elizabeth in Allegheny Commons Park and Downtown at Gateway Center.