Fleming became the first lecturer and instructor in landscape art in the Department of Landscape Art in the College of Agriculture at Cornell University in 1904, the third such program in the United States after Harvard (1900) and the University of Massachusetts (1902).
He served as head of the department from 1906 to 1915 and in 1925 was appointed as University Landscape Advisor to Cornell.
In addition, he served with Warren H. Manning (1860–1938) and others on a comprehensive campus plan for Cornell.
For 30 years Fleming and his associates had an extensive residential design practice all over the country, including estates in Belle Meade, Tennessee, and the design of Cheekwood Botanical Garden and Museum of Art, a 100-acre (0.40 km2) estate where Fleming guided the design of the landscape, architecture, and interiors.
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