He opposed World War One from an internationalist position and rallied to the Russian Revolution in 1917. In 1919, he became a founding member of the Workers (Communist) Party (now known as Communist Party USA) and was part of its leadership from its earliest days.
In the factionalised CPUSA in the 1920s Cannon was responsible for the International Labor Defense organisation from which he built a power base. His followers were loosely organised in the so-called Foster-Cannon faction which looked to "native" American workers in the unions.
In 1928, Cannon read a critique of the direction of the Communist International written by Trotsky which the Comintern had mistakenly circulated. He was convinced of the arguments, and attempted to form a Left Oppositionand within the W(C)P. This resulted in his expulsion. He then founded the Communist League of America with Max Shachtman and Martin Abern, and started publishing The Militant. It declared itself to be an external faction of the W(C)P.
Following the collapse of the Comintern in the face of Nazism in Germany they concluded with Trotsky that the Comintern could not be reformed and embarked on a struggle to build a new International and new parties. This led Cannon and the CL to fuse with AJ Muste's American Workers Party and later, with their augmented forces, to join the Socialist Party of the USA as a faction. This led to an internal struggle with a faction which opposed fusing with the Socilaist Party and which went on to form the Revolutionary Workers League, led by Hugo Oehler. In 1937 having recruited large numbers of people from the SPUSA's youth group, the Young People;s Socialist League, they left the SPUSA and formed the Socialist Workers Party. Cannon became its first secretary.
In 1940 Cannon's co-leader of the SWP, Max Shachtman, left with a large part of the membership to form the Workers Party. Another blow was suffered during World War Two when Cannon was jailed under the Smith Act along with other SWPers. Even in jail however his influence on the SWP was strong and he wrote to change the party line on the Warsaw Rising.
Following the war Cannon resumed leadership of the SWP, but this role declined as he left the post of national secreatary in 1953, and retired to California in the mid-1950s. He took no part in the various factional disputes that developed between 1963 and 1965, except to decry certain factionalism among his erstwhile supporters in a document entitled Do Not Strangle The Party. He died in 1975 in retirement.
A profuse revolutionary journalist, many of his articles have been collected in a series of books, the best known of which are Notebook of an Agitator and The Struggle for a Proletarian Party.