Manning was born in Reading, Mass and began work in his father's horticultural nursery. From 1888 to 1896 he worked in the Olmsted's Brookline office, supervising the firm's planting plans (including the World's Columbian Exposition and Biltmore). Manning founded his own office in 1986 and worked on range of large estates in a style not far removed from that of Andrew Jackson Downing. FletcherSteele and Dan Kiley were apprentices in his office.
Manning also had a keen interest in city planning, learned from the elder Olmsted. He planned numerous parks and park systems (eg for Milwaukee, Minneapolis, St Paul, Louisville, Cincinnati) and argued the case for a 'National Plan for America'. He was a founder member, with ten others, of the American Society of Landscape Architects in 1899.