Born in Hoboken, New Jersey, Koehler was raised in Boston, where his father was a curator at the Boston Art Museum. Koehler began working for the Olmsted Brothers firm as a young man in 1905, where he combined his love of nature with his artistic tendencies, becoming a skilled illustrator and designer. During his long career with the Olmsted Brothers firm, Koehler worked on several notable projects, both public parks and private estates. He established himself as the firm’s plantsman, with great knowledge about trees, and worked on the horticultural enrichment for projects including Branch Brook Park in Newark, New Jersey, parks in the Baltimore system, Washington Park Arboretum in Seattle, Washington, and Fort Tryon Park in New York, where he developed the Heather Garden with James Dawson. His expertise was also an asset to the planning for Bok Tower Sanctuary in Florida and for many of the Long Island and Newport estates. He helped to rejuvenate many of the plantings around Fairsted, the Olmsted Brothers office in Brookline, Massachusetts. Koehler retired in the 1940s, though he continued to advise the practice on plant materials. Koehler published extensively on horticulture, with a particular focus on trees. He is remembered in Boston for creating nametags for tree species in Boston Common and in Marlboro, Massachusetts, where he had made his home. His last years were spent at the Deutsches Altenheim retirement home in West Roxbury where he died in 1951 at age 85.