Hugo Gellert attended art school at The Cooper Union for the Advancement of Science and Art and the National Academy Museum and School of Fine Arts. As a student, he designed posters for movies and theater, and also worked for Tiffany Studios.
As Paul Buhle pointed out: "Gellert reached his artistic turning point when he attended the National Academy of Design and won a prize trip in 1914 to Paris. He intended to enroll at the Académie Julian, but instead was drawn to the sweeping style of contemporary commercial art, specifically the Michelin tire posters. After a walking tour across Europe, he returned to New York and drew anti-war cartoons for the Hungarian American daily Elöre, while making his living in commercial lithography."
Gellert travelled to Europe and witnessed the outbreak of the First World War in 1914. On his return to New York City he began to contribute drawings to the Hungarian-American workers' movement's newspaper, Elöre. He also had his work published in The Masses and during this period became friendly with a group of anti-war journalists and cartoonists such as Max Eastman, Floyd Dell, John Reed, Art Young, Boardman Robinson, Robert Minor, Lydia Gibson, K. R. Chamberlain, Henry J. Glintenkamp, George Bellows and Maurice Becker.
Hugo Gellert caused a political storm when Elöre published his cartoon, Out of the War, in February 1916, that showed an armless veteran being spoon-fed. His anti-war cartoons were also published in other left-wing journals. James Wechsler has pointed out: "Hugo Gellert... is perhaps more infamous for his passionate commitment to leftist political agitation than for his contribution to American art, but Gellert strongly disavowed any distinction between the two. He professed that, for him, political agitation and art were the same thing."
In 1917 Hugo's brother, Ernest Gellert, also a socialist, was drafted into the military but refused to serve on the grounds that he was a conscientious objector. He died of a gunshot wound while imprisoned at Fort Hancock, New Jersey. The army claims his death was a suicide but the circumstances are suspicious. Gellert fled to Mexico to avoid conscription but still continued to provide ant-war cartoons for left-wing newspapers and magazines.
After the USA declared war on the Central Powers in 1917, left-wing journals came under government pressure to change its anti-war policy. When The Masses refused to do this, the journal lost its mailing privileges. In July, 1917, it was claimed by the authorities that articles by Floyd Dell and Max Eastman and cartoons by Art Young, Boardman Robinson and Henry J. Glintenkamp had violated the Espionage Act. Under this act it was an offence to publish material that undermined the war effort.
Henry J. Glintenkamp fled the country for Mexico but the others stood trial in April, 1918. Floyd Dell argued in court: "There are some laws that the individual feels he cannot obey, and he will suffer any punishment, even that of death, rather than recognize them as having authority over him. This fundamental stubbornness of the free soul, against which all the powers of the state are helpless, constitutes a conscious objection, whatever its sources may be in political or social opinion." The legal action that followed forced The Masses to cease publication. After three days of deliberation, the jury failed to agree on the guilt of the defendants.
The second trial was held in January 1919. John Reed, who had recently returned from Russia, was also arrested and charged with the original defendants. Dell wrote in his autobiography, Homecoming (1933): "While we waited, I began to ponder for myself the question which the jury had retired to decide. Were we innocent or guilty? We certainly hadn't conspired to do anything. But what had we tried to do? Defiantly tell the truth. For what purpose? To keep some truth alive in a world full of lies. And what was the good of that? I don't know. But I was glad I had taken part in that act of defiant truth-telling." This time eight of the twelve jurors voted for acquittal. As the First World War was now over, it was decided not to take them to court for a third time.
In 1918 the same people who produced The Masses, including the editor, Max Eastman, went on the publish a very similar journal, The Liberator. Hugo Gellert was chosen to draw the cover of its first issue in March, 1918.
In 1922 the journal was taken over by Robert Minor and the American Communist Party. and in 1924 was renamed as The Workers' Monthly. Many of the people who contributed to the The Masses and the original Liberator, were unhappy with this development and in 1926, they started their own journal, the New Masses. Over the next few years Gellert was a major contributor to this magazine. In 1928, he created a mural for the Worker's Cafeteria in Union Square.
Hugo Gellert was a great friend of John Reed, who died of typhus while in Moscow in 1920. Gellert wrote: "The heroism of the American revolutionary vanguard, the doffed struggles of the workers and farmers in spite of jail, tear-gas and bullets are the source of a new, vigorous art movement in America worthy of the tradition of John Reed - poet and brilliant journalist - a pioneer in American working class culture, a hero and a martyr of the victorious Russian worker's revolution." In 1929 he joined with William Gropper, Jacob Burck, Anton Refregier and Louis Lozowick, to establish the first John Reed Club. The group held classes and exhibitions in New York City. Later, these clubs were formed all over the country.
Despite his left-wing activism Gellert worked as a staff artist at The New Yorker Magazine. In 1932, Gellert was invited to participate in a mural exhibition at the Museum of Modern Art, and submitted a political mural about the connection between industrialists and underworld criminals, entitled, Us Fellas Gotta Stick Together - Al Capone. The museum attempted to censor the mural, along with the murals of William Gropper and Ben Shahn. Other artists threatened to boycott the exhibition over the censorship and they were eventually successful in restoring them to the show. However, they were not reproduced in the catalogue.
In 1933 Gellert illustrated Karl Marx's Capital in Lithographs, and in 1935, he wrote a Marxist, illustrated satire called Comrade Gulliver, An Illustrated Account of Travel into that Strange Country the United States of America. This was followed by Aesop Said So (1936).
After the Second World War Gellert became increasingly concerned with the growth of the extreme right in the United States. His attacks on Joseph McCarthy led to him being called before the House of Un-American Activities Committee. Gellert refused to answer any questions and as a result was blacklisted.
In 1954, Gellert established the Art of Today Gallery in New York City with Rockwell Kent, Maurice Becker and Charles Wilbert White, for blacklisted artists. Gellert served as the gallery's secretary until it closed in 1957. As Paul Buhle explained: "During the 1950s-1970s, he continued to contribute his art widely, on racism, militarism, and the cause of unionism."