Enoch Pratt was a wealthy Baltimore merchant and major benefactor of many Baltimore institutions, including the First Unitarian Church of Baltimore, the Sheppard Pratt Hospital, and of course the Enoch Pratt Free Library. He began to build a mansion for himself and his wife at Monument Street and Park Avenue in 1844. Coincidentally, this is the same year that the Maryland Historical Society was founded, an institution that years later would acquire the building for its collections.
Enoch Pratt was a prosperous hardware merchant, railway and steamship owner, and banker, and originally his new house was three-stories with a basement. In 1868, notable Baltimore architect Edmund G. Lind added a fourth floor, probably in order to keep Pratt in step with the "Mansard" roof trend in Victorian architecture. A new marble portico also was added at the time. The portico had been commissioned by the Clerk of the United States House of Representatives for a mansion in Washington that he ultimately could not afford to build, and Pratt gladly took it off the designers' hands and attached it to his Monument Street residence.
Pratt died in 1896 without any children. He was survived by his wife, who remained in the house until her death in 1911. Soon thereafter, Mary Ann (Washington) Keyser purchased the building for use by the Maryland Historical Society, which has owned the building since 1919.