Canadian-born US labour organizer and radical. In the state of Washington he joined the Industrial Workers of the World (IWW), serving as secretary and organizer for several IWW locals in the Seattle area where he assisted longshoremen and loggers. In 1916 he became secretary-treasurer of the IWW's Everett Defense Committee and assisted in the 1917 acquittal of Wobblies charged with murder after the ‘massacre’ of November 1916.
Mahler was born in Chatham, Ontario, Canada. He left home in 1910 to find work in the Canadian West, working as a riverboat pilot and logger until he emigrated to the USA in 1915. During World War I he participated in IWW lumber and copper strikes that tied up war materials, even though he believed more in organization of workers than antiwar activities. He was indicted along with 100 IWW leaders in Chicago, Illinois, in 1917 and, in 1918, was convicted of three counts of espionage and sedition, sentenced to 12 years in prison, and fined $20,000. He remained in prison until President Calvin Coolidge commuted the sentences of all IWW inmates still incarcerated in 1923. Named secretary of the IWW's General Defense Committee after his release from prison, he became general secretary-treasurer in 1931, but resigned the following year over disputes concerning IWW direction. He continued organizing work outside of IWW auspices, moving to New York, and then organizing the Kentucky Miners Defense Committee in 1937. His last public appearance was picketing the New Republic in 1948 to protest an unsympathetic portrayal of Joe Hill.