After graduating high school, Dunne attended the College of St. Thomas in St. Paul, a private, Roman Catholic institution.

Dunne went to work on the Northern Pacific Railroad as an electrician and joined the International Brotherhood of Electrical Workers (IBEW).

[5] Dunne was promoted in the IBEW to vice president in charge of organizing for the Pacific District, which included Washington, Oregon, Idaho, and British Columbia.

Dunne led the Butte IBEW local to join the strike and shut down copper production as fifteen thousand workers walked off the job.

In retaliation, mine owners hired Pinkerton agents to aid strike breakers, and Montana governor Sam V. Stewart declared martial law.

When the more moderate factions of miners reached an agreement with the mining companies and returned to work, radical groups, including the socialist Industrial Workers of the World, took control of the strike.

He published admiring editorials and political cartoons about the Bolshevik Revolution and reprinted reports from Russia by John Reed.

[8] In February 1918, the Montana legislature passed a series of bills designed to bolster the war effort and squelch opposition.

Among these was a sedition act that made it a crime to speak, write, or publish anything "disloyal, profane, violent, scurrilous, contemptuous, slurring or abusive" about the U.S. government, soldiers, or symbols.

[9] The Montana Council of Defense, which had been established to coordinate county war efforts, took it upon itself to enforce the sedition act.

[12] On September 13, 1918, soldiers raided the offices of the Butte Daily Bulletin searching for posters that Dunne had allegedly printed in support of a national strike.

[13] Dunne's opposition to the council made him popular in Butte, and Silver Bow County elected him to the Montana House of Representatives in November 1918.

[12] He also sponsored a resolution demanding that U.S. troops be withdrawn from Russia, where they supported the anti-Communist side in the Russian Civil War.

Dunne's trial for sedition took place during his legislative term, causing his absence from nearly half of the legislature's working sessions.

Despite his long tenure, Dunne was always regarded as a bit of a loose cannon in the Communist movement, as historian Theodore Draper recounts:

"[18]Dunne was a delegate to the ill-fated August 1922 convention of the Communist Party of America (CPA), held in Bridgman, Michigan.

[22] In 1923, Dunne was expelled from the American Federation of Labor for his communist political views and activity in organizing the so-called "left wing" of the labor movement through the Communist Party's trade union affiliate, the Trade Union Educational League.

[4] He was elected a member of the Comintern's Organization Bureau at the 5th Enlarged Plenum of the Communist International, held in March 1925.

[23][24] In 1934 he went so far as to author a polemic pamphlet for the Communist Party against his brothers and their comrades entitled Permanent Counter-Revolution: The Role of the Trotzkyites in the Minneapolis Strikes.

In 1937, Dunne returned to the western United States as the organizer of Communist Party district 33, which included Idaho, Montana, Wyoming, and Utah.

[25] In 1939, Dunne worked to oppose U.S. entry into World War II on behalf of the Communist Party until 1941 when Germany invaded the Soviet Union.

He worked in the Navy shipyards, joined the merchant marine, and eventually took a job as a cook at a base in the Aleutian Islands.

In 1947, Dunne published a book, The Struggle Against Opportunism in the Labor Movement – For a Socialist United States, deriding the leadership of the CPUSA for abandoning the principles and methods Marxism-Leninism in favor of collaborating with the Democratic Party.