Sadler was born in London, England, to Walter Dendy Sadler, a master of domestic genre painting. After graduating from the University of Reading, England, he worked several years for the Barr & Sugden Nursery in London. He travelled to the United States in 1913 to work for the Olmsted Brothers firm, an affiliation he would continue for two decades. In 1922, Sadler briefly worked for Wayne E. Stiles, opening his firm’s New York office. Rejoining the Olmsted Brothers two years later, he collaborated with James Frederick Dawson and George Gibbs, Jr. on the plans for Palos Verdes and the Bixby Estate (Rancho Los Alamitos) in California as well as the Washington State Capitol grounds. Opening his own firm in 1934, Sadler worked through the Depression on large estates in Beverly Hills and Bel Air, including the Hilda Boldt Weber estate, Casa Encantada. Beginning in 1939, he designed several Modernist landscapes for public and private, large-scale housing projects including Wyvernwood, Estrada Courts, and Lakewood City. Elected a Fellow of the ASLA in 1923, Sadler continued creating landscapes for schools, businesses, and residences until his death.