Despite his opposition to conscription, Coleman enrolled with the Australian Imperial Force in February 1918 and served with the Army Service Corps in France and England.

Coleman toured the United States of America as a lecturer in 1920, and returned as secretary of the renamed Australian Clerical Association, a post he retained until 1922.

Coleman's standing in the union movement led him to be selected as the Labor candidate for the new seat of Reid in 1922, an endeavour in which he was successful.

Despite some negative publicity, during which he was called before a royal commission to testify with regard to apparent bribes made to some Labor members, Coleman easily held his seat in 1925 and 1929.

Admitted to the bar in 1933, he became president of the New South Wales (Federal) branch of the Australian Labor Party, but died unexpectedly on 25 May 1934 while in a car at Concord.