The building was constructed by the firm of George A. Fuller, who trained as an architect but made his mark as the creator of the modern contracting system in building construction. His firm had supervised construction of the Rookery, and later built New York's Flatiron Building with Burnham in 1902. The Monadnock Block was built as a single structure but was legally two buildings, the Monadnock and the Kearsarge, named for Mount Monadnock and Mount Kearsarge in New Hampshire. Work was completed in 1891. The Monadnock, which Root called his "Jumbo", was his last project; he died suddenly while it was under construction.