Born in Muscatine, Iowa, Downer graduated from Princeton University in 1905, earning a degree in Civil Engineering. Downer went to work for the Aluminum Company of America before he was appointed principal assistant engineer for the newly-formed Bronx Parkway Commission in 1907. In 1911 he became the Commission’s secretary and chief engineer and oversaw the development of the Bronx River Parkway in New York from 1912 - 1925. In 1923 he was appointed chief engineer of the newly formed Westchester County Park Commission (WCPC) and managed the development of the Saw Mill River, Hutchinson River, and Cross County Parkways, as well as public parks, beaches, marinas, and other amenities, such as Playland Amusement Park in Rye, New York (now Playland). Between 1923 and 1932, while working for the WCPC, Downer collaborated on the design of the southern section of the Taconic State Parkway (the Bronx Parkway Extension) in New York.
Downer resigned from the WCPC in 1934 and spent the next seven years as an engineering consultant to John D. Rockefeller, Jr., advising on projects including the Palisades Interstate Park in Alpine, New Jersey; Colonial Williamsburg in Virgina; and Forest Hill Park in Cleveland, Ohio. He served as a member of the board of design for the 1939 World’s Fair, and as a consultant for the National Capital Park and Planning Commission. Downer consulted on the design of the National Airport in Arlington, Virginia (now Ronald Reagan Washington National Airport) and, beginning in 1942, served as chief engineer of Idlewild Airport in New York City (now John F. Kennedy International Airport).
Downer was bestowed an Honorary Doctor of Science degree from Columbia University in 1931, and a scholarship was established in his name at Princeton University. He was named a Corresponding Member of the American Society of Landscape Architects in 1932. Downer passed away at his home in Larchmont, New York, and is buried at Oakdale Memorial Gardens in Davenport, Iowa.