In 1927 Dedman stood unsuccessfully as the Country Party candidate for Upper Yarra in the Victorian Legislative Assembly. However, the Country Party helped defeat a bill to establish a milk board in the Legislative Council and, as a result, he joined the Labor Party. Between 1932 and 1934, he unsuccessfully contested one federal and two state seats. He was a councillor on Upper Yarra Shire Council from 1926 to 1939 and its president in 1931 and 1937. In 1938, he studied Keynesian economics at the University of Melbourne.