Having studied at the School of Mines at Columbia College, Greenleaf both practiced and taught as a civil engineer for fifteen years before leaving the Columbia faculty to begin designing landscapes in the New York region. In the late 1890s, he received his first major commission, the request of James B. Duke to create parkland out of the flat acreage surrounding his Somerville, New Jersey estate. This important project simultaneously drew upon his engineering expertise and introduced him to large scale planting design. Over the next 20 years, Greenleaf’s popular Country Place Era landscapes were noted for their graceful proportions and elegant transition from formal, often geometric spaces closer to the residence, to the more naturalistic areas at further distance. In response to clients’ desires to achieve immediate effects in the landscape, Greenleaf became expert at moving large trees, a subject about which he wrote for the trade. His most significant project was the design for Green Garden, George D. Pratt’s estate at Killenworth on Long Island. Greenleaf served on the National Commission of Fine Arts for nine years and had a significant influence on many historic sites in Washington, DC, most notably the Lincoln Memorial. He also advised on American war cemeteries in Europe and national parks in America.